Re: Let's make planes that can't fly......
US planes were excellent Typhoon and Mustang, yes, great.
..and the Mustang was initially designed and built for the RAF to fulfill British requirements for a long-range fighter. At that time the Spitfire and Hurricane were short range metropolitan defence aircraft, which is what they'd been designed for, and once the Battle of Britain was over, the RAF needed something with a longer range that could take on the Luftwaffe over Europe.
The Mustang wasn't up to much with the original Allison engine, especially at altitude, but fitting a Merlin transformed it. Years ago I knew a member of the ground crew responsible for its initial trials in the UK, who described the Allison as a beautifully made car engine and who claimed that he was part of the team that replaced it with a Merlin during the initial trials.
The USAAF only got Mustangs later: remember that they thought that escort fighters weren't needed because B-17 formations had enough guns to deal with any fighters they might meet.