The first B-57As were largely identical to the Canberra B.2s with the exception of more powerful Armstrong Siddeley Sapphire engines of 7,200 lbf (32 kN) of thrust instead of Rolls-Royce Avons, also license-built in the US as Wright J65s. In addition, canopy and fuselage windows were slightly revised, the crew was reduced from three to two, wingtip fuel tanks were added, engine nacelles were modified with additional cooling scoops, and the conventional "clamshell" bomb-bay doors were replaced with a low-drag rotating door originally designed for the XB-51.