The B-66 was developed for the United States Air Force (USAF) and is heavily based upon the United States Navy's A-3 Skywarrior, a heavy carrier-based attack aircraft. Originally, officials intended for the aircraft to be a simple development of the earlier A-3, taking advantage of being strictly land-based to dispense with unnecessary naval features. However, due to the USAF producing extensive and substantially divergent requirements, it became necessary to make considerable alterations to the design, leading to a substantial proportion of the B-66 being original rather than derived from the A-3. The B-66 retained the three-man crew arrangement of the US Navy's A-3; differences included the incorporation of ejection seats, which the A-3 had lacked.