The Armstrong Whitworth Argosy is a British post-war transport/cargo aircraft. It was the final aircraft to be designed and produced by the aviation company Armstrong Whitworth Aircraft. Although given different internal design numbers, the AW.650 civil and AW.660 military models were, for most practical purposes, the same design and shared the "Argosy" name.

Development of the Argosy originates in the AW.66, a proposed twin-engined military transport that was designed with British Air Ministry's Operational Requirement 323 (OR323) in mind. While Armstrong Whitworth terminated work on the AW.66, it decided to go forward with a civilian-oriented derivative of the design, designated AW.65, as it was judged to be commercially viable. The AW.65 was redesigned to use four Rolls-Royce Dart turboprop engines, and thus was re-designated as the AW.650. On 8 January 1959, the first Argosy conducted the type's maiden flight. In December 1960, the type received Federal Aviation Administration (FAA) type certification, enabling the initial civil version, referred to as the Series 100, to enter civil service across most parts of the world.

In Britain, military planners took interest in the Argosy and released a new specification for a militarised variant, designated AW.660. First flown on 4 March 1961, this model featured double the range of the Series 100 and otherwise differed by an alternative door arrangement, which was largely to facilitate paratroop operations. Furthermore, an improved civil variant, the Series 200, was introduced at the behest of airline British European Airways (BEA). First flown on 11 March 1964, this model featured a new wing incorporating a fail safe structure, being stronger and lighter than its original counterpart. The Argosy was operated by both the Royal Air Force (RAF) and various civil operators across the globe for numerous years. The type was withdrawn from RAF service during 1978, while the last Argosy was retired from civil operations during 1991.

The development of the Argosy can be traced back to the development of Operational Requirement 323 (OR323) by the British Air Ministry. During 1955, a specification was issued based upon OR323, which sought a medium-range freight aircraft that would be capable of lifting a maximum payload of 25,000 lb (11,340 kg), while also possessing a range of 2,000 mi (3,200 km) when carrying up to 10,000 lb (4,500 kg). British aviation manufacturer Armstrong Whitworth Aircraft took interest in this specification and decided to allocate members of its design team to the task of developing a suitable aircraft to meet its demands. Initially, design efforts were focused upon a twin-engine design intended for military use, which was internally designated as the AW.66.

As it was recognised that the aircraft held sales potential within the civil market, a civilian-oriented variant, designated AW.65, was also designed alongside the military design; the AW.65 principally differed from the AW.66 via the installation of full-section doors at each end of the fuselage to enable rapid loading and unloading operations. However, a lack of available finance contributed to the company's decision to abandon all work towards meeting the military requirement; despite this setback, Armstrong Whitworth had already decided to proceed with developing the civil variant as a private venture. The company believed that the type would have significant appeal to the growing short-haul air freight sector of both the European and American markets. At that time, no other aircraft had been purpose-designed for such a purpose. As work continued, the AW.65 was extensively redesigned, including the adoption of four Rolls-Royce Dart turboprop engines; the resulting aircraft being designated as the AW.650.

On 8 January 1959, the first Argosy performed its maiden flight. It appeared during that year's Farnborough Airshow, by which point five aircraft were flying, having cumulatively amassed about 400 flight hours between them. Certification-related testing of the type was reportedly completed during September 1960. In December 1960, the Argosy received type certification from the American Federal Aviation Administration (FAA), authorising the aircraft's entry to commercial service. 10 of the initial civil version, the Series 100, were built; construction of these aircraft had commenced months prior to receiving certification so that deliveries could commence as soon as possible.

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.

Early on, civil operator British European Airways (BEA) had shown open interest in the Argosy, the company viewed the aircraft as a potential replacement for its existing piston engined freighters; however, evaluations of the Series 100 soon found that its payload capacity would not allow for the type to operate profitably. Early on, as a measure taken to reduce design costs, the wing of the Argosy had been based on that of the Avro Shackleton, a maritime patrol aircraft that was developed and built by another entity within the Hawker Siddeley Group; in order to satisfy BEA's requirements, a new wing was designed which shared the same aerodynamic design, but benefitted from a more modern fail-safe structure rather than the safe-life design used on the earlier wing. This change resulted in a wing that was both stronger and lighter, but was also no longer limited in terms of its fatigue life. The revised version, designated as the Series 200, also featured several other improvements, including the adoption of enlarged cargo doors, integral wing fuel tanks and a modified landing gear arrangement. The Series 200 reportedly had better handling than the older Series 100, although some aerodynamic refinements were required during testing.