Re: Not sure what the big deal is
"the P-51 Mustang"
Apparently the British Air Purchasing Commission or whatever it was called, spent a whole lot of time with NAA, passing on some very highly specific operating specifications before they even arranged to order a prototype. So I think you could say that the P-51 Mustang was for all intents and purposes a British-US collaboration. Ironically, the only other two aircraft the US produced that came close to having similar performance - the Grumman Hellcat and the Chance-Vought Corsair: IMHO the best strike fighter US companies developed - were US Navy specifications.
Also in the Mediterranean the USAAF flew Beaufighters until Black Widows became available.