While the RAF had lost interest in any acquisition of the original AW.66 design, the service still needed to procure a replacement for its obsolete piston engined transport fleet, including the Vickers Valetta and Handley Page Hastings. During 1959, the British Air Ministry released a new specification that called for a military derivative of the AW.650, the envisioned aircraft was intended to serve in multiple roles, including as a medium-range transport, paratroop and supply aircraft. The resultant design, which was designated "AW.660", was significantly different from the AW.650. Changes included the sealing of the nose door, its location being instead occupied by the radome of a weather radar unit, while the rear doors were substituted for by an alternative clamshell style which incorporated an integral loading ramp, while a stronger cargo floor was also installed. A pair of additional doors were fitted, one each on the starboard and port sides, which enabled paratroopers to exit. The military Argosy was powered by an arrangement of four Rolls-Royce Dart 101 turboprops and possessed twice the range of the civil Series 100. From July 1960, the second Argosy Series 100 was used to flight-test the new clamshell door design. On 4 March 1961, the first of the 56 Argosies destined for RAF service performed its first flight.