megan,
Wiki tells me much more than I previously knew before about the P-51. Seems that it was first built on contract for the wartime British Purchasing Commission (like the Vultee Vengeance), and later adopted by the USAAC. We called it the "Mustang", and used it for the low and medium level roles to which it was restricted by the limited height capability of the Allison engine.
The installation of the "Merlin" in the P-51B/C transformed it into a world-beater; its pilots reckoned it as good as (or better) than the Spitfire (hotly disputed by the Spitfire people - I cannot comment as I never flew a Mustang - but then, as an old Spitfire hand, can anything be better than a Spitfire?) Though I must admit that its much longer range made it far superior as an escort. (Was it Goering, who in the last days in Berlin said, that when he saw the "red noses" of the P-51 escort over Berlin, he knew the war was lost!) The P-51D, with the Packard Merlin, was just the icing on the cake.
"That's what the RAF told the USAAF, Danny". Their informant was talking through his hat, IMHO. Which British aircraft ever used them? (if you count out the original batch).
Danny42C.