A percentage of the changes made were a result of the USAF's requirement for the B-66 to perform low-level operations, the complete opposite of the US Navy's A-3, which had been developed and operated as a high-altitude nuclear strike bomber. However, aviation authors Bill Gunston and Peter Gilchrist attribute many of the design changes to have been made "merely to be different", being driven by an intense rivalry between the two services; they conclude that "an objective assessment might conclude that 98 per cent of the changes introduced in the RB-66A were unnecessary". Both the fuselage and wing were entirely redesigned from scratch, rather than simply de-navalised. The A-3 was powered by a pair of Pratt & Whitney J57 turbojet engines, whereas the B-66 used two Allison J71 engines; Gunston and Gilchrist note that this engine swap "offered no apparent advantage", generating less thrust and being more fuel-hungry than the J57 engine which was also already in USAF use.