F-19

F-19 is the designation for a hypothetical US fighter aircraft that has never been officially acknowledged, and has engendered much speculation that it might refer to a type of aircraft whose existence is still classified.

Since the unification of the numbering system in 1962, U.S. fighters have been designated by consecutive numbers, beginning with the F-1 Fury. F-13 was never assigned to a fighter due to superstition, though the designation had previously been used for a reconnaissance version of the B-29. After the F/A-18 Hornet, the next announced aircraft was the F-20 Tigershark. The USAF proposed the F-19 designation for the fighter, but Northrop requested the "F-20" instead. The USAF finally approved the F-20 designation in 1982. The truth behind this jump in numbers is Northrop pressed the designation "F-20" as they wanted an even number, in order to stand out from the Soviet odd numbered designations. Despite this, the designations F-17, F-21, and F-23 were not skipped.

Throughout most of the 1980s, "F-19" was thought to be the designation of the stealth fighter whose development was an open secret in the aerospace community. When the actual aircraft was publicly revealed in November 1988, its designation was revealed to be F-117.

Another rumor was that F-19 is really the designation of some other super-secret project, one so black that it will not be revealed for many years.

One more version was part of a deliberate plot by the Air Force to confuse Soviet intelligence by hoodwinking them into expending so much effort in trying to find out information about a plane that does not exist.

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Wikipedia