Hawker Siddeley HS 748 Paper ended most military manned-aircraft development in the UK, and Avro decided to re-enter the civilian market. The Vickers Viscount had the larger end of the short-haul market, and Avro therefore decided to design a smaller regional airliner powered by two Rolls-Royce Dart turboprop engines, intended to replace the many DC-3 Dakotas that were by then reaching the end of their economic lifespan. Original plans were for a 20–30 seat aircraft with a similar configuration to the Fokker F27, but talks with potential customers soon led to a low-wing 40-seat design being chosen for the 748 project. Avro was not
Vickers Valiant deterrent to the Soviet Union from acts of aggression in Europe. In April 1948, the Air Staff issued a specification with the designation B.9/48 written around the Vickers design, which was given the company designation of Type 660; an 'Instruction to Proceed' was received by Vickers on 16 April 1948. In February 1949, two prototypes of the aircraft were ordered. The first of these was to be fitted with four Rolls-Royce RA.3 Avon turbojet engines, while the second was to be fitted with four Armstrong Siddeley Sapphire engines as the Type 667. On 18 May 1951, the first prototype, serial