The British approached North American about license-building P-40s. North American said, "We can build you something better." Thus was the Mustang born.They got in early because Curtis figured out how to convert the P-36 Hawk to the P-40 Warhawk while North American, Republic, Lockheed and all the rest were still fumbling in the dark.
If you're interested, Air Classics magazine has been telling a VERY detailed story of the early Mustangs over the last six or so issues.