Excuse me, but has NO ONE corrected the info on this aircraft all this time. It's an NA-91 aircraft, 92 of which went to the RAF under Lend Lease and 58 to the US ( 1 to USN and 57 to USAAF) as "P-51" (no letter suffix --- already mentioned) "Apaches." They officially became Mustangs in the USAAF in July, 1942.
The RAFs were named "Mustang Mk IA" --- most of the USAAF's P-51 Mustangs became Tac-R aircraft after cameras were added in 3 (maybe more) locations in the US, and a couple years later were redesignated "F-6As." They kept their four 20 mm Hispano Mk II cannons (license-built by Oldsmobile), even after cameras were added. Two of the NA-91s became the XP-51B Mustang prototypes when Merlins and 4-bladed props were adapted to the airframe, along with cooling system and other changes. The one that went to the USN became a "chase plane" and sorta "never was heard of again."
The NA-97 A-36A Mustang followed the NA-91 aircraft and were NEVER officially named ANYTHING but "Mustang," although some of the pilots and groundcrewmen in the 27th FBG in the MTO wanted them renamed "Invaders," but Douglas already had that name on their excellent A-26 Invader.
What else can I tell you? OH, and I have scans of historical documents, most of which came from the USAAF and NAA, so they're PRIMARY sources, which naturally I trust over anything you can read in Wikipedia or on websites.
Please reply to me if I've made any errors in my clarification above --- I'm only human and mistype stuff alllll the time!
Thanks.
MustangTMG