The P51 was designed and built by North American, to British specifications, for the RAF. They flew it and discovered that the Allison engine was by far the Mustang's worste drawback. It was suggested that the Mustang airframe, combined with RR's Merlin engine would be a superb aircraft.....turns out they were right.
It's high altitude performance, sweet handling and range made it an obvious choice as a bomber escort, something the 8th AF desperately needed.
So, though undoubtably designed for the Brits, the Mustang can only be called an American fighter, powered by a Brit engine, and flown, mostly, by Americans ( though by no means exclusively).
A tricky question really, but I cant see any justification for calling the P51 Mustang anything but American.
<S> Blue