Several other minor differences were also present between early US-built and UK-built Canberras. These changes included the canopy and fuselage windows being slightly revised, the crew being reduced from three to two, the adoption of wingtip fuel tanks, 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. As a result of these changes, the bomb bay of the US-built aircraft was slightly smaller than its UK-built counterparts in order to allow the American aircraft to carry more fuel; however, the overall external shape of the Canberra remained unchanged.