Mustang = North American
(North American -> Rockwell -> Boeing)
P-51 started out as A-36 (Attack); early British request for a fighter to supplant Spits and Hurricanes removed dive brakes, revised the guns, and upgraded the low altitude Allison engines (with a marginally better Allison), did it so well that the US kept most of them, licensed the Merlin engine, and Commonwealth got a bunch of P-40s.
The Brits wanted more P-40s, but Curtis was at capacity. North American had idle plant, so the Brits asked if the could build P-40s under licence, North American said "We can do you better than that" and developed the P-51
Two years later, the USAAF asked NA to develop a ground attack version.