An inquiry to the Air Force Public Affairs office resulted in a return call from the Air Force Materials Command at Wright-Patterson Air Force Base. The F-35 designation was simply chosen, without regard to any sequence of numbers, according to an Air Force spokesman.
This is the most dramatic break in the Tri-Service Designation Systems Fighter series since its inception in 1962 a total of eleven designations. The previous breaks in the post-1962 fighter series were the F-13 and F-19. The former was apparently skipped over due to the myriad of superstitions surrounding the number thirteen. There are several theories as to why the latter was skipped: One story had it being skipped to force the Soviets to waste intelligence resources to pursue a secret project many thought this was to be the designation of the stealth fighter project (actually it was the F-117).
The next question will be what happens with the future fighters that will be procured. Assuming the F-32 designation would have gone to Boeings X-32 (the losing competitor in the JSF fly-off), the numbers 24 through 31, as well as 33 and 34, a total of ten designations, are unaccounted for. Some of these designations may be assigned to aircraft that the United States has acquired. One of the gaps between the F-111 and the F-117 in the older series is filled by a fighter known as the YF-113. The plane is actually the Russian MiG-23 Flogger, and the YF-113s in American hands probably were acquired by various means for training and evaluation. Yet something has to be put into the pilots log books, and so a designation was created, although it was not released to the public.
One other reason that designators are skipped is due requests from a manufacturer or government agency. The X-49 designator was skipped for the Dragonfly, a Canard Rotor/Wing flying vehicle. Since it is a 50/50 hybrid between a helicopter and airplane, the Defense Advanced Research Projects Agency (DARPA), requested it be designated X-50. This is not the first time. In World War Two, Fisher requested that the P-73 and P-74 designations be skipped to give them the P-75 designation so as to create a catchy ad slogan (The French 75 in World War One; The P-75 in World War Two). A similar public relations move could be the reason the F-19 designation was skipped for Northrops F-20 Tigershark (to give the impression that it was of a new generation of fighters).
Other times, a designation will be altered for simplicity. The A-37 Dragonfly was based on the T-37 Tweet, an early primary jet trainer. It was a simple decision to switch the T-37 to A-37, since it was a heavily modified version of the trainer, and would normally be called the AT-37.
This dovetails with the last reason for an unusual designator: Saving money and effort. This is why the AH-64D comes in versions with and without the millimeter-wave fire-control radar. The version without the radar was going to be designated the AH-64C, but the decision to go with just the AH-64D designation saved a little cash in the slim defense budgets in the 1990s.
There have been a number of other weird designations recently, like the KC-767, which is planned to the KC-135Es currently in service (the logical Tri-Service Designation would be KC-41). Two others are the FB-22, a planned strike variant of the F-22 Raptor (an apparent similarity to the FB-111 as a derivative of the F-111), and the AL-1A Airborne Laser, where the AL reportedly stands for Airborne Laser. Harold C. Hutchison
The Lockheed-Martin X-35 has been selected as the winner of the JSF (Joint Strike Fighter) competition, and designated the F-35. This is ironic, since the last plane in the F-series was the YF-23 Black Widow, the loser in the Advanced Tactical Fighter competition (the winner became the F-22). The question is obvious: Why not designate the X-35 as the F-24 or F-25?