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.^[1] There have been a number of theories put forth to explain this omission, but none have ever been confirmed.

The most prevalent theory in the 1980s was that "F-19" was the designation of the stealth fighter whose development was an open secret in the aerospace community. When the actual aircraft was publicly revealed in 1988, it was called the F-117 Nighthawk. There seems to be no evidence that "F-19" was ever used to designate the Nighthawk, although the National Museum of the United States Air Force website does include the entry "Lockheed F-19 CSIRS (see F-117)" as of 2011.^[2] Another theory suggests that F-19 was the designation applied to the Have Blue technology demonstrator which led to the development of the F-117.