We couldn't have built the pony, because we were too busy building spits, typhoons, mossies, lancs, stirlings, wellingtions, hallifax, tempests, meteors, and many other fine aircraft. As well as inventing the Jet, Radar and cracking the enigma code.
Plus we were under air attack, blockade via the Uboat and generaly fighting for survival.
We built a damn good engine though that made the pony what it was.
Also we bombed by night and therefore had no real need of a daylight escort fighter. The pony was supposed to supplement our existing fighter force I believe and only realy proved itself as the wars finest escort fighter for USAAF daylight bombing missions.
So US designed and built, British power, proven as a daylight escort fighter for USAAF
As for me I'm quite pleased we had Radar and the Spitfire and Hurricane otherwise the War in Europe may never have been there for you guys to help win.
