In September 1940, 150 aircraft designated NA-91 by North American were ordered under the Lend/Lease program. These were designated by the USAAF as P-51 and initially named the Apache although this name was dropped early-on for Mustang. The British designated this model as Mustang IA. They were equipped with 4 long barrelled 20 mm Hispano-Suiza Mk II cannon instead of machine guns. A number of aircraft from this lot were fitted out by the USAAF as photo reconnaissance aircraft and designated F-6A. The British would fit a number of Mustang Is with photographic reconnaissance equipment as well. Also, two aircraft of this lot were fitted with the Packard built Merlin engine and were designated by North American as model NA-101 and by the USAAF initially as the XP-78, but re-designated quickly to XP-51B.