The Armstrong Whitworth A.W.38 Whitley was a British medium/heavy bomber aircraft of the 1930s. It was one of three twin-engined, front line medium bomber types that were in service with the Royal Air Force (RAF) at the outbreak of the Second World War. Alongside the Vickers Wellington and the Handley Page Hampden, the Whitley was developed during the mid-1930s according to Air Ministry Specification B.3/34, which it was subsequently selected to meet. In 1937, the Whitley formally entered into RAF squadron service; it was the first of the three medium bombers to be introduced.

Following the outbreak of war in September 1939, the Whitley participated in the first RAF bombing raid upon German territory and remained an integral part of the early British bomber offensive. In 1942 it was superseded as a bomber by the larger four-engined "heavies" such as the Avro Lancaster. Its front-line service included maritime reconnaissance with Coastal Command and the second line roles of glider-tug, trainer and transport aircraft. The type was also procured by British Overseas Airways Corporation as a civilian freighter aircraft. The aircraft was named after Whitley, a suburb of Coventry, home of Armstrong Whitworth's Whitley plant.

In July 1934, the Air Ministry issued Specification B.3/34, seeking a heavy night bomber/troop transport to replace the Handley Page Heyford biplane bomber. This combination bomber/transport was part of the RAF's concept of fighting wars in distant British Empire locations, where the aircraft would fly into the theatre of action carrying troops and then provide air support.

John Lloyd, the Chief Designer of Armstrong Whitworth Aircraft, chose to respond to the specification with the AW.38 design, which later was given the name Whitley after the location of Armstrong Whitworth's main factory. The design of the AW.38 was a development of the Armstrong Whitworth AW.23 bomber-transport design that had lost to the Bristol Bombay for the earlier Specification C.26/31.

Lloyd selected the Armstrong Siddeley Tiger IX radial engine to power the Whitley, which was capable of generating 795 hp (593 kW). One of the novel features of the Whitley's design was the adoption of a three-bladed two-position variable-pitch propeller built by de Havilland; the Whitley was the first aircraft to fly with such an arrangement.

As Lloyd was unfamiliar with the use of flaps on a large heavy monoplane, they were initially omitted from the design. To compensate, the mid-set wings were set at a high angle of incidence (8.5°) to confer good take-off and landing performance. Flaps were included late in the design stage, but the wing incidence remained unaltered; as a result, the Whitley flew with a pronounced nose-down attitude when at cruising speed, resulting in considerable drag.

The Whitley holds the distinction of having been the first RAF aircraft with a semi-monocoque structure in its fuselage, which was built slab-sided to ease production. This replaced the tubular construction method traditionally employed by Armstrong Whitworth, who instead constructed the airframe from light-alloy rolled sections, pressings and corrugated sheets. According to aviation author Philip Moyes, the decision to adopt the semi-monocoque fuselage was a significant advance in design; many Whitleys surviving severe damage on operations.

In June 1935, owing to the urgent need to replace biplane heavy bombers then in service with the RAF, it was agreed to produce an initial 80 aircraft, 40 being of an early Whitley Mk I standard and the other 40 being more advanced Whitley Mk IIs. Production was initially at three factories in Coventry; fuselages and detailed components were fabricated at Whitley Abbey, panel-beating and much of the detailed work at the former Coventry Ordnance Works factory, while wing fabrication and final assembly took place at Baginton Aerodrome. During 1935 and 1936, various contracts were placed for the type; the Whitley was ordered "off the drawing board" - prior to the first flights of any of the prototypes.