North American's follow-on to the BT-9 was the T-6 Texan trainer, of which 17,000 were built, making it the most widely used trainer ever. The twin-engine B-25 Mitchell bomber achieved fame in the Doolittle Raid and was used in all combat theaters of operation. The P-51 Mustang was initially produced for Britain as an alternative to the Curtiss P-40 Warhawk, which North American had declined to produce under licence. The derivative A-36 Apache was developed as a ground attack aircraft and dive bomber. This was done, in part, to keep the airframe in production as the US Army Air Corps had not yet decided to purchase the type as a fighter.