Also keep in mind that the time a plane BEGINS development, is not the time a plane enters service. Many planes had long and complicated development cycles... So while the P-47 saw service before the P-51, the P-38 saw service after the P-40.
Consider the P-39 earned an order for a prototype in October of 1937, and had the prototype flying by April 1938.
The XP-39E was an up-engined version designed to get more out of the plane. They were tested in Feb 1942, and re-designated P-76. 4,000 were to be built, but production was cancelled 3 months later in favor of a redesigned air frame to go WITH the more powerful engine.
So the P-63 was born. First flights Dec 1942 and Feb 1943.
Goes from P-39 to P-73 to P-63, chronologically speaking.
I just liked that example because it's a good illustration that the number didn't really imply order of inclusion.
