US fighter designations

Has anybody else ever wondered how the US arrives at its designations for its fighter aircraft?

It used to be quite simple. P (later F) followed by a sequential number indicating the sequence in which the design was registered.

This was fine until they got to the early F one hundreds. The F100, 102, 106, 111 followed in sequence, with the missing numbers generally being ascribed to designs which were never built or completed, or failed to enter service.

Then, along came the F-4, sometime back in the 1950’s. The SR-71 was originally the XF-12, then YF-12A. These were followed by F-5, F14, 15,16,17, etc and this continues with big gaps up to the F-22 and F-35.

Fine. So why was the F-117 designated as such, right in the middle of all the other numbers? Anybody got any ideas? And can you think of any others which were out of sequence and why?

Regards

Wombat