Due to the great urgency placed on delivering the B-57A Canberra, the initial phase of manufacturing performed by Martin used a minimum-change formula to expedite production; thus the first B-57As to be produced were largely identical to the Canberra B.2. As such, more than 30 changes that had been requested by the Wright Air Development Center (WADC) were rejected on program schedule or cost grounds. One noticeable exception was the adoption of more powerful Armstrong Siddeley Sapphire engines capable of producing 7,200 lbf (32 kN) of thrust, which were license-built in the United States as the Wright J65. The Sapphire-based J65 powerplant had been selected in place of the British-built Canberra's Rolls-Royce Avons as the USAF had placed an emphasis on operations within hot climates and intended to fly at a slightly higher maximum gross weight than the RAF's operating practices.