I recently wondered this myself. The best I could determine from Wikipedia [1] of what's known publicly is that it's seemingly an artifact of when the project began. The theory being that it was allotted a designation before the "Century Series" ended in the 1960s. (Further suggesting that other secret projects for F112-F116 were also allotted designations.)

[1] https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#Desig...

At least with small arms, often the "missing" model numbers are either 1)one-off, special purpose purchases for a single service, or 2)something uninteresting. For example, at least 1 issued "pistol" prior to the M9 was a flare gun. The M3 Carbine, ostensibly the "predecessor" to today's M4, was a WW2/Korea night-sight-equipped M1 Carbine.

With aircraft, I assume the trend is similar, with the "missing" numbers being trials aircraft or testing designs.

There are F-105 and F-111 that are ground attack aircraft like the F-117. I think part of it was to hide the capabilities of the aircraft from other nations. Cf. SEAL Team Six

The numbering system was reset in 1962 as part of a bigger change, unifying aircraft designations across all the service branches. Some aircraft designated prior to this kept their designation, while others changed. Soviet aircraft obtained by the US for test and evaluation were often given designations >100 so they could be referenced internally without giving away what they really were, as the evaluation program was classified.

As for the F-117; fighters are generally regarded as sexier and more attractive than attack, bombers, cargo or other types. By calling it a fighter the USAF reckoned that recruitment would be easier and also that it's real purpose would be obfuscated. This is also the reason for the number; by giving a high number they indicated that this was an airplane designed prior to 1962 which never became anything.

F-104 is Vietnam era, as far as I remember. But then the F-14 was developed in the 80s. I would certainly buy into the theory that the point is to confuse the enemy.

The F-104 is almost Korean war era; its maiden flight was in 1954, it entered active service in the late fifties and remained operational at least into the early eighties in some countries - my uncle was conscripted as a mechanic on the Starfighter for the Royal Norwegian Air Force in 1979-1980. They were phasing in the F-16 at the time, so probably retired the F-104 shortly thereafter.

> my uncle was conscripted as a mechanic on the Starfighter for the Royal Norwegian Air Force in 1979-1980

Fascinating! Did you get any cool tours as a child?

-I was two years old at the time and only know this from photo albums, I'm afraid; he got out of the air force at first opportunity and started working on the 727/737 for a commercial carrier instead.

I still never fail to get an interesting story or two from his AF days whenever we meet, though - trusting stuff like the Starfighter to a bunch of twenty-somethings is basically ASKING for lots of close shaves growing into very tall tales...

F-14 was developed in the late 60's and reached IOC in 1973.