 Air Ministry official, Air Chief Marshal Sir Wilfrid Freeman (Chief Executive at the Ministry of Aircraft Production - MAP) lobbied vociferously for Merlin-powered Mustangs, insisting two of the five experimental Mustang Mk Xs be handed over to Carl Spaatz for trials and evaluation by the U.S. Eighth Air Force in Britain.In this Freeman was greatly assisted by the US Assistant Air Attaché to London, Thomas Hitchcock Jr. After sustained lobbying at the highest level, American production started in early 1943 of a North American-designed Mustang patterned after a P-51 Mustang prototype originally designated the XP-78 that utilized the Packard V-1650-3 Merlin engine replacing the Allison engine.The pairing of the P-51 airframe and Merlin engine was later designated P-51B for the model NA-102 (manufactured at Inglewood, California) or P-51C for the model NA-103 (manufactured at a new plant in Dallas, Texas from summer 1943).