Originally Posted by Gun Grape

We didn't.

The F-19, most people agree is the F-117. Seems that the AF didn't want to have a "secret plane" that already had a plastic model and a computer game named after it. The 1/48 scale model put out by Testors/Italeri became the best selling model of the 80s and possibly the best selling model of all time. Even though it wasn't accurate at all.

The other theory being that it is the designation for the "aurora"

The F-21 were Kfir's leased by the USN and USMC for their aggressor squadrons between 1985-1989.

The F-24 was the original designation for the Superbug. Until they decided that the upgrade had to be a mod vice a new plane

Some of the others were used for US desgnations of foreign planes for evaluation and the X planes. Like the X-29 forward swept demonstrator and the X-31 Enhanced Maneuverability Demonstrator (Vector)

Every once in a while we seem to "Skip a few" to get back on track.