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　　Beginning in 1919, the USAAS (US Army Air Service) assigned simple alphanumerical designations

　　to all its aircraft. The basic system, consisting of letters designating type or mission followed

　　by a sequential number ("Design Number"), was continued by the USAAC, USAAF, and USAF, even if the used type designation

　　letters changed significantly several times in history. In 1962, this designation system became mandatory

　　for use by all U.S. military services. The current designation system is explained in detail on this site in the article

　　about Current Designations Of U.S. Military Aircraft, and for details about the earlier designation systems

　　you may refer to this Wikipedia article.

　　There are (at least) two publicly available printed sources which attempt to give

　　complete lists of USAF (and predecessors) and post-1962 Joint aircraft designations. These

　　sources are:

　　For the latest designators, the references of choice are

　　With these sources as a reference, there are very few gaps left in the designation listings. Most of these

　　can either be explained as deliberate gaps, or have been "closed" by further research. This article presents a list of

　　designations which cannot be found in any of the above sources, together with the current knowledge about the use/non-use

　　of these designators. For the sake of completeness, designations which are already explicitly explained as "skipped" in [1] or [2]

　　are also included.

　　Note: For each current series, an entry labeled is provided

　　to indicate which design number would allocated next in that series, if numbers are assigned in sequence. However, since a few years,

　　allocating arbitrary numbers has become increasingliy common.

　　A-1

　　A-1 was not assigned in the attack series, because the Cox-Klemin XA-1 (designated in the pre-1924 A-for-Ambulance series)

　　was still in existence when the first attack aircraft (Douglas XA-2) was designated in 1924. There was also a Fokker A-2

　　ambulance, but this was already out of service in 1924.

　　A-8

　　The designation A-8 was never used as such, because the "A-8" slot is incorrectly used by the

　　BAE/McDonnell Douglas AV-8 Harrier VTOL aircraft. For more

　　information about the AV-8 designation, see article about

　　Non-Standard DOD Aircraft Designations.

　　A-11

　　Officially, the A-11A designation was reserved but never assigned (for unknown reasons) in the 1986/88 time frame. There are a few rumours about a secret

　　"A-11 Astra" stealth attack aircraft, but these are unconfirmed, and probably have no basis in reality. Even if such an aircraft actually existed,

　　the use of the A-11 designator for it would be extremely unlikely.

　　A-13

　　Not used for superstitious reasons (see F-13).

　　A-14

　　The designation A-14A was briefly reserved to an Embraer A-29B Super Tucano evaluated by the US Navy. But the MDS was changed to

　　A-29B (to match the Brazilian designation) before the aircraft was taken up. Therefore the A-14 slot will remain effectively unassigned.

　　The next available design number in the A-series is A-15.

　　B-71

　　The proper pre-1962 B-series ends with B-70. Although the number 71 in the designation SR-71 followed on from B-70, the SR-71

　　was never planned to be a bomber, and the designation B-71 was never reserved, let alone allocated, to it. For some background

　　information on the SR-71 designation, see article about

　　Non-Standard DOD Aircraft Designations.

　　B-72 and higher:

　　The missile numbers SM-68 to GAM-87 are included in the B-series of sources [1] and [2], with gaps at B-74, B-79, B-81, B-82, B-84,

　　B-85 and B-86. However, this is strongly misleading, because only missiles TM-61, SM-62, GAM-63, SM-64, SM-65 and GAM-67 did actually

　　carry B-designations originally. The numbering sequence for missiles forked from the B-series at #68, so any missiles numbered 68

　　and higher never used a B-designation. All of the gaps (plus higher numbers up to 92) were indeed allocated to missiles and rockets.

　　For details about these missile designations, see article about

　　Pre-1963 Designations Of U.S. Missiles And Drones.

　　The next available design number in the B-series is B-3. The B-3 designation has been used informally to refer to proposed future

　　bomber concepts, but has never been allocated. In 2016, the designation B-21 was allocated to the LRS-B program. With this allocation,

　　the use of a numbering sequence for bombers has effectively ended.

　　C-13

　　The C-13 designation was never assigned. This may be the first example of the skipping of -13 for superstitious reasons.

　　Although -13 was not skipped in any other pre-1962 series, this has now become standard practice since the omission of F-13

　　in the post-1962 fighter series (see F-13).

　　C-136

　　The C-136 designation had originally been skipped and reserved for use by the US Navy (see also H-36/38). The slot was later

　　used by the USAF, when it allocated the designation YC-136 on 19 July 1956 to a proposed modification of the

　　Fairchild C-123B Provider. The YC-136 was to

　　be a single prototype aircraft to test various improvements for the C-123B design.

　　The YC-136 was apparently cancelled before it was built.

　　C-138

　　The C-138 designation had originally been skipped and reserved for use by the US Navy (see also H-36/38). Later, it was

　　planned for allocation to the Fairchild F-27

　　(license-built Fokker F-27 Friendship), if that type were acquired by the

　　USAF. This did not happen, and therefore the C-138 designation remained unused. It is not known, why a designation was reserved for

　　the F-27 before any firm plans to acquire the aircraft were actually made.

　　C-139

　　The designation SC-139 was requested on 18 October 1954 for a variant of the

　　Lockheed P2V-7 Neptune, but the designation

　　was disapproved on 23 November 1954. C-139 was not reassigned to another project.

　　C-143

　　The pre-1962 C-series ends at C-142. The XC-143 designation was requested for the

　　Curtiss Model 200, but the request

　　was turned down on 12 February 1962. Instead, the aircraft became the X-19, and still later (on 18 September 1962) the X-19A.

　　In December 2005, the pre-1962 C-series was revived, when the designation C-143A was allocated to a

　　Bombardier (Canadair) "Challenger" 604

　　aircraft, which had been acquired by the U.S. Coast Guard. Unofficial information indicates that there has been some DOD-internal

　　confusion if C-42 (the nominal next number in the post-1962 C-series) was really available for allocation. The result was the continuation

　　of the older C-series, because the C-143 slot was definitely available (see also paragraph on C-42 in new C-series).

　　The "old" C-series is currently used in parallel with the "new" one (it is unclear which one will be used for the

　　next allocation). The next available number in the "old" series is C-148.

　　C-13

　　Not used for superstitious reasons (see F-13).

　　C-16

　　The C-16 designation was never officially assigned to any aircraft. However, it was tentatively reserved no less than four times before it

　　was finally cancelled for good. The first reservation, dated 27 November 1973, was for "YC-16" for an unidentified Boeing aircraft, but

　　this reservation is marked as "not used". On 13 March 1975, the designation C-16A was reserved for the

　　deHavilland Canada DHC-6. However, this

　　aircraft was eventually designated as UV-18A, and C-16

　　remained unused. On 17 December 1975, C-16 was again put "on hold" for the Air Force. This third reservation was officially cancelled

　　on 30 April 1981, shortly after the fourth and final reservation for C-16 was made on 13 April 1981. The latter was for the

　　"C-X" aircraft (which eventually became the

　　C-17A Globemaster). A formal request for allocation

　　of an MDS designation, forwarded on 3 September 1981 by the USAF Nomenclature Office to HQ USAF for approval, says:

　　However, only one day later this letter was cancelled and replaced by the following:

　　There was obviously a reason to skip C-16, but unfortunately it was not written down. It is reported, however,

　　that the design number 16 was skipped because of "concerns over confusing the plane with the F-16 during the stress

　　of high combat radio traffic".

　　Interestingly, when Boeing worked in the 1978/80 time frame on a four-engined YC-14 derivative for the C-X

　　competition, some drawings were labeled with "C-16". However, this was simply a Boeing in-house label reflecting the anticipated

　　designation of the C-X, and not related to any officially reserved C-16 designation.

　　C-30

　　The designation C-30 was never officially assigned. It was reserved (most probably after a verbal request at some time in the 1988/89

　　time frame) for the USAF office with symbol "SAF/AQQX" (SAF/AQQ is the office symbol for the Assistant Secretary of the Air Force

　　(Acquisition), Directorate of Global Reach; AQQX designates the Programs, Budget & Congressional Division of AQQ). Nomenclature

　　records do not contain any written request, let alone official allocation of the C-30 designation. Neither are there any releasable

　　records on "C-30" at SAF/AQQX. The reservation was most likely dropped, but usage for a classified purpose can't be ruled out completely.

　　The "KC-30" label, which was used by Northrop Grumman for their proposed tanker version of the EADS/Airbus A330, is

　　not an official DOD designation. The same is true for the KC-30A designation used by the Royal Australian Air Force for its

　　A330-based tankers.

　　C-34

　　The C-34 designation was never assigned, because it was skipped at the request of the U.S. Army when the latter requested

　　a designation for what became the UC-35A

　　aircraft. On 9 February 1996, the Army's Product Manager for Fixed Wing Aircraft requested a designation for said aircraft:

　　On 19 March 1996, the request was forwarded by the USAF Nomenclature Office to HQ USAF, which in turn approved the skipping of

　　C-34.

　　C-36

　　At some time in the March/May 1996 time frame, the designation YFC-36A was reserved for the U.S. Air Force. The only available detail

　　is that this aircraft was to be a four-engined type. However, the "YFC" prefix (cargo aircraft converted to a prototype interceptor)

　　is very unusual, and (together with the 1996 time frame and the "4 engines") strongly suggests that YFC-36A was temporarily reserved

　　for what became the YAL-1A "Airborne Laser" aircraft.

　　For more information on the non-standard YAL-1A designation, see article about

　　Non-Standard DOD Aircraft Designations.

　　C-39

　　Unclassified DOD nomenclature records contain no data at all on C-39, not even a non-descript reservation as is the case for

　　a few other numbers, e.g. A-11 and C-30. There is one report stating that C-39A was the originally planned

　　designation for what eventually became the U.S. Navy's C-40A Clipper.

　　A possible reason would be that C-39 was skipped to avoid confusion with the CT-39 still in Navy service. However, available documentation

　　does not support this report. The request by the Naval Air Warfare Center Aircraft Division for a new MDS for the aircraft, dated 10

　　February 1998, doesn't request any specific model number. The letter simply shows "C-___A", with the number to be assigned by the USAF.

　　The request was approved by HQ USAF with the allocation of model number 40. Apparently no specific reason was given by HQ USAF to the

　　Nomenclature Office why C-40A was allocated. However, given the skipping of V-14 and C-34 for a very similar reasons, it's not unlikely

　　that the "confusion with CT-39" report is indeed true, even if this reason was not recorded in any public documents.

　　C-42

　　The C-42 designation was not assigned. There is a very popular small sport plane, which is marketed world-wide under the "C42"

　　(or "C-42") label: the German Comco Ikarus C42. A DOD source

　　said unofficially, that there was some concern about potential legal issues if C-42 would be used for an MDS, and that this

　　number was therefore skipped.

　　C-43, C-44

　　The designations C-43 and C-44 were skipped to avoid potential confusion with the existing T/CT-43 and T-44 designators.

　　According to unofficial information from DOD, there is an informal policy in effect to avoid duplication of "well-known" numbers.

　　However, C-45 was not skipped (KC-45A is the official MDS for the KC-X tanker program), and it remains unclear why T-45

　　should be any less "well-known" than T-43 and -44. One possible explanation is that both the T-43 (Boeing 737) and T-44

　　(Raytheon/Beech King Air) are transport-type airframes (the T-45 is a two-seat jet), and that it was therefore avoided to assign

　　numbers 43 and 44 to other transport aircraft as well.

　　The next available design number in the C-series is C-47. However, the designation C-47T (a turboprop-powered upgrade of the old

　　C-47 Skytrain transport of WW II vintage) was still listed as a non-retired MDS as of late 2018. Therefore it is not unlikely

　　that the C-47 slot will be skipped for the next C-series allocation.

　　E-7

　　The original E-7 designation was reserved in August 1981 and cancelled in January 1982. It was a Boeing 707 which was then allocated

　　the basic designation C-18 (with the E-7 becoming the EC-18)

　　due to its possible multi-mission roles.

　　The Royal Australian Air Force operates the Boeing 737 AEW&C

　　aircraft as the E-7A Wedgetail, but until 2022 this was not a U.S. DOD designation. However, the USAF has firm plans to procure

　　variants of the Wedgetail as replacement for its E-3 Sentry AEW fleet. In November 2022, the E-7A designation

　　was added to the U.S. DOD MDS database, closing the gap at E-7.

　　The next available design number in the E-series is E-12.

　　F-109

　　This designation is usually attributed to the Bell Model D-188 supersonic VTOL

　　fighter project of the late 1950s. However, while Bell and/or the ARDC (Air Research and Development Command) twice requested the

　　YF-109 designation for this project (in January and October 1958), both requests were officially turned down by HQ USAF.

　　A few years earlier (most likely in 1955), F-109 was also requested for the two-seat interceptor version of the F-101 Voodoo

　　(which became the F-101B in the end). However, this request (which is somewhat inexplicable anyway, because the next number in line at

　　that time was F-108) was apparently cancelled at an early stage, because no written documentation about it exists in USAF nomenclature

　　records (the only available first-hand source is a preliminary "Standard Aircraft Characteristics" sheet). Still other reports link

　　the F-109 designation to a proposed operational version of the Ryan X-13 Vertijet VTOL demonstrator, but there is no firm evidence

　　available for this claim.

　　All said, F-109 was never officially allocated to any fighter project.

　　F-112 through F-116

　　The pre-1962 F-series ends with F-111. However, ever since the designation of the Lockheed Nighthawk stealth fighter was

　　announced as F-117A, there has been speculation about the "missing" numbers F-112 through -116. It is now known (although

　　it has never been officially acknowledged by the Air Force) that at least some of these numbers have been assigned to

　　secret projects of all sorts, including Soviet aircraft secretly acquired and tested by the USAF. However, the assigments were

　　not made in sequence, so that they can't be regarded as a proper continuation of the pre-1962 F-series. For more information,

　　see article on Cover Designations for Classified USAF Aircraft.

　　F-13

　　The designation F-13 was definitely not used. The Navy's F-14 Tomcat

　　fighter would have received this number, but F-13 was rejected by Grumman and/or the Navy (obviously for superstitious reasons). Since

　　that time (late 1960s), it is an unwritten rule that "-13" designators are always skipped. This eventually also led to the omission of

　　A-13, C-13, G-13, Q-13 and V-13.

　　F-19

　　The F-19 designation was never assigned. The official explanation by DOD was "to avoid confusion with MiG-19", which was generally regarded

　　as very implausible (because so far no numbers had been skipped to avoid clashes with foreign designators). Therefore, it was much

　　speculated whether F-19 was really skipped, and if so, for what real reason. One viable theory was that F-19 was originally allocated to

　　(or at least reserved for) the F-117A Nighthawk, but eventually

　　not used (see also article about Non-Standard DOD Aircraft Designations). The other main line

　　of reasoning was that Northrop specifically requested the F-20 designator for its then new

　　Tigershark (originally designated F-5G) to make it

　　look as "the first of a new fighter generation" (i.e., the "20" series).

　　The truth is in fact a combination of the second idea and the official line. The designation "F-19A" was indeed officially skipped at

　　Northrop's request. They wanted to redesignate the F-5G as F-20A, because they preferred an even number. The Soviet competitors in

　　the export fighter market of the early 1980s all used odd numbers, and Northrop wanted to stand out from these. So the official

　　"confusion with MiG-19"-story is in fact more or less close to the truth, although the phrase is misleading. Nobody would

　　"confuse" an "F-19A" with a MiG-19, especially because the latter was obsolete anyway at that time. To say it again, Northrop

　　didn't want to avoid "confusion" with MiG-19 in particular, but to use an even number to stand out from all the Soviet odd ones.

　　The F-20A designator was approved despite official recommendation by the USAF Standards Branch (at that time responsible for

　　nomenclature assignments) to follow the regulations and use "F-19A".

　　The facts are documented by several letters exchanged between various USAF/DOD offices during the process of

　　requesting and assigning the F-20A designator to Northrop. On 28 October 1982, HQ Aeronautical Systems Divison, USAF (apparently

　　handling the F-5G program for the Air Force) wrote a letter to the USAF Standards Branch to request a new model number for the

　　F-5G on behalf of Northrop Corporation. To quote the relevant part (remarks in [brackets] are by me):

　　On 1 November 1982, this request was in turn forwarded by the Standards Branch to USAF HQ in the Pentagon for approval.

　　However, it was clearly stated that the designation should be "F-19A" instead, to follow existing regulations:

　　(The copy of the letter has a hand-written note at the bottom, saying "Dissapproved [sic] See F-20A folder".)

　　On 17 November 1982, HQ USAF approved the F-20A designation (but not yet the Tigershark name), apparently

　　ignoring the Standards Branch's advice, and introduced the misleading "confusion with MiG-19" phrase:

　　As a side note, the name Tigershark for the F-20A was eventually approved on 30 March 1983. It had

　　originally been requested on 4 September 1981 for the F-5G, but was then rejected "due to a proliferation

　　of popular names for the F-5 aircraft series and the speculative nature of the F-5G venture" [USAF quote].

　　The last sequentially assigned MDS in the F-series was F-23, so technically, the next available design number is F-24. However, the designation

　　YF-24 has apparently been used (most likely "semi-officially") for an as yet undisclosed classified aircraft (referenced in the

　　official USAF biography of Colonel Joseph A. Lanni), so it could be argued that F-25 is next in line. But in any case, there is,

　　to all intents and purposes, no longer a sequence of F-designations.

　　The last two numbers (F-35 and F-47) have been assigned out of sequence, and it can be assumed that any future fighter (e.g. the U.S. Navy's

　　upcoming "F/A-XX") will also get a "special" number in one way or the other.

　　G-1

　　TG-1A was allocated to the

　　Schweizer SGS 2-25 sailplane used by the Air Force Academy.

　　G-2

　　TG-2A was allocated to the

　　Schweizer SGU 2-22E sailplane used by the Air Force Academy.

　　G-3

　　TG-3A and TG-3B were allocated to the

　　Schweizer SGS 1-26B and SGS 1-26E sailplanes

　　used by the Air Force Academy.

　　G-4

　　TG-4A was allocated to the

　　Schweizer SGS 2-33A sailplane used by the Air Force Academy.

　　G-5

　　TG-5A was allocated to the

　　Schweizer SGS 2-32S sailplane used by the Air Force Academy.

　　G-6

　　TG-6A was allocated to the

　　Schweizer SGS 1-34 sailplane used by the Air Force Academy.

　　G-13

　　Not used for superstitious reasons (see F-13).

　　The next available design number in the G-series is G-18.

　　H-36, H-38, H-44

　　In the early 1950s, some even numbers in the C-series (C-134/136/138) and H-series (H-36/38) of USAF aircraft designations were

　　set aside for use by the US Navy, apparently in an attempt to establish a joint designation system for some type categories.

　　However, this scheme was very short-lived, and had been abandoned by 1955. There is no evidence that any of the three C-series

　　numbers or H-36 was ever actually used by the Navy, but the case is a bit more confusing for H-38. The USAF aircraft serial

　　54-4047 is listed in USAF records as a "Sikorsky H-38", allocated to a Navy-led MAP (Military Assistance Program). There are

　　indications that the serial refers to a "static airframe", but otherwise no further information whatsoever about this H-38 has

　　been found so far.

　　In any case, by 1959 the USAF apparently regarded the H-36 and -38 slots as unused. A letter from an Air Force Intelligence

　　office (AFCIN-4F), dated 18 May 1959, requested the allocation of H-36, H-38 and H-44 (which was the next available number at

　　that time) to three secret projects called LONG EARS (H-36), SHORT TAIL (H-38) and BIG TOM (H-44). This letter explicitly

　　mentions that H-36 and -38 had been "set aside but never used by the Navy". All three designations were approved in August 1959.

　　The USAF aircraft serial 59-5926 is listed as a "Bell H-36", but otherwise no information whatsoever is available about

　　the three projects or the associated helicopters.

　　There is a rumour that the Sikorsky XV-2 (Model S-57) design (which was cancelled around 1954/55 before a prototype was

　　built) was originally designated XH-36. While this doesn't sound implausible (the XV-1 and XV-3 were both originally designated in

　　the H-series as XH-35 (→ XV-1) and XH-33 (→ XV-3), respectively), there is no indication whatsoever in very respectable

　　secondary sources (which all list the redesignations of the XV-1 and -3 designs) for an XH-36 → XV-2 redesignation.

　　In fact, as is documented in the preceding paragraphs, the primary source (USAF nomenclature allocation letters) explicitly

　　says that H-36 had been skipped. So it appears that the association of the XV-2 with the XH-36 designation is in error.

　　At best, it was unofficially discussed at some time early in the S-57 project to designate the aircraft as XH-36, but

　　there was no formal effort to reserve, let alone allocate, the H-36 designation for the project.

　　H-42

　　XH-42 was allocated to a Hughes Model 269 for the U.S. Army, which eventually

　　became the YHO-2HU. It was later developed as the TH-55A Osage.

　　H-45

　　H-45A was allocated in January 1962 to a secret Air Force FTD (Foreign Technology Division) project called STEP CHILD. USAF serial

　　62-5980 is attributed to an "XH-45", but otherwise no information is available about the STEP CHILD program or the H-45 helicopter.

　　H-49

　　XH-49A was briefly allocated in 1962 to a Boeing-Vertol Model 107

　　helicopter for the USAF. The designation was changed to XCH-46B before September 1962.

　　H-69

　　When the VH-71A designation was assigned to the Lockheed Martin US-101

　　(winner of the VXX (Presidential Helicopter) competition), the design numbers 69 and 70 in the H-series were skipped (H-70 has since been

　　allocated to the Bell ARH). As has been confirmed by the VXX Program Office, the number "69" was regarded as embarrassing, because that

　　number is also known as a synonym for a certain sexual practice. According to USAF/XPPE, H-69 will not be assigned in the future

　　to any helicopter.

　　The next available design number in the H-series is H-74.

　　K-1

　　The K-series was officially reserved for tanker aircraft between 1955 and (at least) 1977, but no designation

　　was ever allocated in this series. All U.S. tankers were derivatives of other aircraft, and therefore the K-series was

　　cancelled at some time between 1977 and 1986, and only the K-for-Tanker modified mission symbol remains. A purpose-built

　　tanker would most probably receive a "KC" designation in the C-series.

　　The next available design number in the L-series is L-2.

　　O-4

　　The designation ZO-4A (indicating a design in the planning stage) was reserved by the USAF on 28 May 1969. The O-4 was to be a quiet

　　observation aircraft, similar in concept to the Lockheed YO-3A. It seems that one candidate (possibly the only one) for the O-4 requirement

　　was the Wren 460QB, a modified Wren 460B (itself a highly modified derivative of the Cessna 182).

　　In 1969, the USAF proposed the purchase of 28 of these aircraft. However, the O-4 program was terminated in the early 1970s because of budget restrictions.

　　The next available design number in the O-series is O-7.

　　P-73

　　P-73 was never officially assigned. The generally accepted reason is that the USAAF skipped P-73 and P-74 because the

　　Fisher Division of General Motors requested the "special" number 75 for their

　　XP-75 Eagle escort fighter. However, the XP-73

　　designation was at least proposed for (but not actually allocated to) the

　　Hughes D-2, which was eventually designated as XA-37.

　　P-74

　　P-74 was never assigned (for details, see P-73).

　　P-1

　　The designation P-1 was definitely not used, apparently out of convenience to use numbers P-2/3/4/5 to redesignate the

　　P2V, P3V, P4Y, and P5M, respectively (see also article on Aircraft Redesignations in 1962).

　　P-6

　　The designation P-6 was never assigned, even if it is frequently quoted as post-1962 designation of the

　　Martin P6M Seamaster. However, official DOD aircraft nomenclature

　　forms of 1962 say "P-6 skipped for the Navy". This was most likely done to avoid any future confusion with the P6M.

　　The next available design number in the P-series is P-10.

　　Q-7

　　In late 1953, the WADC (Wright Air Development Center) requested the redesignation of the QB-17 drone to Q-7A, Q-7B and Q-7C (presumably

　　representing different drone configurations, possibly the QB-17G, -17L and -17N). The stated reason were problems and delays in the

　　production and support of the drones, because they were too easily confused with unmodified B-17 aircraft. However, HQ ARDC

　　(Air Research & Development Command) did not concur with this reasoning, and turned the request down in March 1954. Because

　　the Q-7 designation was not re-used otherwise, the #7 slot in the Q-series remained unassigned.

　　Q-8

　　In 1948, a few remaing Culver PQ-8A Cadet drones were possibly redesignated as Q-8A. However, these were discarded soon after, and

　　the Q-8 designation became available for use in the regular Q sequence.

　　In late 1953, the WADC (Wright Air Development Center) requested the redesignation of the QF-80 drone to Q-8A, Q-8B and Q-8C (presumably

　　representing different drone configurations, possibly QF-80A, -80C and -80F). The stated reason were problems and delays in the production

　　and support of the drones, because they were too easily confused with unmodified F-80 aircraft. However, HQ ARDC (Air Research &

　　Development Command) did not concur with this reasoning, and turned the request down in March 1954. Because the Q-8 designation was not

　　re-used otherwise, the #8 slot in the Q-series remained unassigned.

　　Q-11

　　In July 1959, the WADC (Wright Air Development Center) requested the allocation of the designation XQ-11 to a so called "F-108 Airborne

　　Target". This referred to a projected high-performance aerial target for evaluation of the whole F-108 interceptor weapons

　　system (WS-202A). However, USAF Headquarters did not approve the request, stating that a specific designation for the target would not

　　be necessary at such an early stage of the program. The Q-11 designator was not used otherwise afterwards, and therefore the #11 slot in

　　the Q-series remained unassigned.

　　Q-13

　　Not used for superstitious reasons (see F-13).

　　The next available design number in the Q-series is Q-30.

　　R-17

　　The R-series continued the old F-series, which had reached F-15 by 1947. Reference sources usually mention only a single

　　post-1947 designation, the XR-16. However, when the USAF first planned to acquire its own batch of the Lockheed U-2,

　　following the CIA's lead, it used the designation R-17 for the aircraft. In the end, this designation was dropped, and

　　the USAF aircraft were also designated U-2.

　　R-2 and higher:

　　The only official designation allocated in the proper sequence of the R-series so far is TR-1, and even this is a

　　non-standard designation. The NASA's

　　Lockheed ER-2 aircraft is listed as "R-2" in some sources,

　　but this is an error. The "ER" stands for "Earth Resources", and the ER-2 designation was assigned by NASA and not by the DOD.

　　The TR-3A designation, which was rumoured to be allocated to a secret stealth battlefield surveillance platform

　　(which almost certainly doesn't exist), is purely fictitious.

　　The next available design number in the R-series is R-2.

　　S-1

　　The designation S-1 was definitely not used, apparently out of convenience to use number S-2 to redesignate the S2F

　　(see also article on Aircraft Redesignations in 1962).

　　The next available design number in the S-series is S-4.

　　The S-for-Spaceplane series is in potential conflict with the S-for-Anti-Submarine series. So far, the only spaceplane designation

　　is MS-1A, which created no duplication, because the S-1 slot was not used in the anti-submarine series. Whether any new spaceplane

　　MDS would use the S-2 or the S-4 slot is unknown.

　　T-4, T-5

　　Both designations were skipped to use T-6A Texan II for the new USAF/Navy JPATS Trainer to honour the AT-6/T-6 Texan

　　of WW II fame. After the subsequent allocation of T-7, it's very unlikely that these gaps will be filled by later training aircraft.

　　T-48, T-49

　　A memo from HQ USAF (which confirms the allocation of the T-1A designation to the Beech Jayhawk) to the USAF Nomenclature Office,

　　dated 28 February 1990, includes the following paragraph:

　　The designation YT-48A is generally associated with a proposed development of the Cessna T-37 as a replacement for the

　　USAF's cancelled T-46A. It seems that the USAF used their reserved T-series MDS for this proposal. The T-49A designation, which was

　　reserved for the Navy, was apparently never taken up. In the end, a common JPATS aircraft was selected, which was designated

　　T-6A Texan II (see also T-4/T-5 above).

　　During 2003, the designations T-48 and T-49 were newly assigned (see Addendum to DOD 4120.15-L).

　　Therefore it seems that the earlier reservations had been cancelled, because a re-use of reserved but never allocated and later cancelled

　　designations is actually "legal" in the MDS system.

　　T-50

　　The T-50A MDS designator is not allocated to any current U.S. military aircraft, but has been officially reserved for the

　　Lockheed Martin/KAI Golden Eagle military trainer, which is marketed under the "T-50" company designation.

　　Although the U.S. military currently has no plans to procure the Golden Eagle, it was apparently regarded as too confusing to

　　have T-50A eventually assigned to a different model.

　　The next available design number in the T-series is either T-55 or T-8, depending on whether the next number will be allocated in

　　the sequence carried over from the pre-1962 numbering, or the new one started in 1962.

　　U-12, U-13, U-14, U-15

　　The designations U-12 through -15 were definitely not used, apparently out of convenience to use number U-16 to redesignate the SA-16

　　(see also article on Aircraft Redesignations in 1962).

　　I don't know why a 4-number gap was approved just to keep a single number unchanged.

　　The next available design number in the U-series is U-29.

　　V-13

　　Not used for superstitious reasons (see F-13).

　　V-14

　　In May 1973, the U.S. Army and NASA requested an MDS designation for the forthcoming

　　Bell Model 301 tilt-rotor demonstrator aircraft.

　　On 30 May 1973, the designation XV-14 was allocated. However, on 7 August 1973, a change of this designation was

　　requested to avoid confusion with the Bell X-14B VTOL aircraft. To quote the most important parts of this request:

　　The letter continues to explain at some length how easily the two designations could be confused, especially because

　　both aircraft are built by companies both known shortly as "Bell" (Bell Helicopter Company XV-14 and Bell Aerospace

　　Company X-14B). Reference is made to the potential of serious errors if the service and maintenance records

　　of the two aircraft are somehow mixed up. The letter ends with:

　　The redesignation request was approved on 22 August 1973, and the aircraft became the XV-15.

　　V-17

　　XV-17 was assigned to the Army on 24 May 1973. No details are available.

　　V-19

　　V-19A was reserved on 25 February 1977 for the Navy, but the reservation was cancelled on 8 December 1980. A note on the nomenclature

　　record card says:

　　Given that not even the responsible Navy officer knew what the V-19A designation was intended for after only 3 years, it looks unlikely

　　that we will ever find out more about the "V-19".

　　V-21

　　The designation PV-21 was reserved on 15 December 1983 by the Navy for a blimp (non-rigid airship) with tilt rotors.

　　In 1983/84, the USN conducted a "Patrol Airship Concept Evaluation Study" (PACES). This included flight-testing of an experimental

　　tilt-rotor blimp during mid-1983. The PV-21 designator (indicating a patrol mission) was possibly reserved either for the PACES demonstrator

　　or a planned tactical development, like the Battle Surveillance Airship System studied as a follow-on project to PACES.

　　The next available design number in the V-series is V-26.

　　X-23

　　The X-23A designation is generally attributed to the

　　Martin Marietta SV-5D PRIME unmanned lifting body

　　reentry test vehicle, but available USAF nomenclature records show that X-23A was never actually assigned.

　　On 16 November 1965, the designation X-23A was requested for the SV-5P

　　[sic!] vehicle, which is known to have been designated as X-24A

　　in mid-1967 (see next paragraph). The vehicle description accompanying the designation request of 1965 clearly describes the SV-5P as a

　　low-speed (Mach 2 to landing) manned lifting-body aircraft. However, in a letter dated 15 December 1965, the request was disapproved

　　for the reason that the subject aircraft was unmanned (at that time, the aircraft designation system was still used as originally

　　intended in 1962, i.e. for manned aircraft only)! This appears to be very weird indeed, but apparently there was a severe misunderstanding

　　regarding the nature of the research aircraft at the office which had to approve the designation.

　　In late 1966, the offices responsible for the USAF's lifting body reentry programs again pondered the question how to designate

　　the test vehicles. After a stillborn proposal to introduce a completely new designation category for gliding reentry vehicles,

　　it was decided that the best way to go was to request the designations X-23A for the unmanned SV-5D PRIME and X-24A for the

　　manned SV-5P. X-24A was accordignly requested and approved, but it appears that no actual request for X-23A was ever sent

　　to the nomenclature office. Reasons are unknown, but maybe it was realized that an MDS request for an unmanned vehicle was futile,

　　especially when the rejection of the 1965 request for X-23A explicitly said that unmanned aircraft need no designation. Whatever the

　　reasons, the designation X-23A was never even requested for, let alone allocated to, the SV-5D PRIME vehicle.

　　X-39

　　X-39 was reserved on 23 April 1997 for the FATE

　　(Future Aircraft Technology Enhancements) program of the AFRL (Air Force Research Laboratory). However, no

　　formal written request to allocate X-39 to FATE was put forward, and therefore X-39 remained officially unassigned.

　　X-52

　　The designation X-52A was requested in 2006 for a program to test

　　active aeroelastic wing technology, but was disapproved because of

　　possible confusion with the B-52 series. Instead the designation X-53A was allocated to the program.

　　X-58

　　The X-58 designation has been skipped. The designation XQ-58A was assigned to the

　　Kratos Valkyrie

　　UAV built for the AFRL (Air Force Research Laboratory), and it is practically certain that the number 58 was chosen to fit into

　　the X-series even though an "XQ"-designated vehicle should get a number in the Q-series.

　　X-67

　　Just like X-58, the X-67 slot will almost certainly be skipped after the allocation of XQ-67A to the

　　General Atomics OBSS (Off-Board Sensing Station) UAV.

　　The next available design number in the X-series is X-68.

　　The next available design number in the Z-series is Z-5.

　　Back to U.S. Military Aviation Designation Systems