In 1927, the British Air Ministry issued Specification 26/27 for a replacement of the elderly Airco DH.9A, designed during the First World War which still equipped the Royal Air Force's General Purpose squadrons. To save time and money, the specification called for the use of a high proportion of DH.9A components, (as the RAF still held large stores of DH.9A spares), while it encouraged the use of an all-metal structure. A large number of types were tendered to meet this requirement from most of the major British aircraft manufacturers, including Westland, who submitted the design that later became known as the Wapiti. Westland had an advantage in that it had carried out the detailed design work for the DH.9A, and was already a major contractor for the DH.9A.
The Wapiti was a conventional single-engined equal span two-bay biplane with slight stagger. It had tandem open cockpits and a fixed main undercarriage plus tail-skid. The forward fuselage was of metal tube structure with aluminium and fabric covering, while the rear fuselage was of fabric covered wooden construction. The wings and tail were standard wooden DH.9A components, although later models replaced the wooden parts with an all-metal structure. The Wapiti was powered by a single Bristol Jupiter radial engine, and its crew of two were armed with a forward firing Vickers machine gun and a Lewis gun for the observer, while it could carry up to 580 lb (264 kg) of bombs under the wings and fuselage. It was also fitted with radio and photographic cameras, and like the DH.9A before it, could carry a spare wheel for operations in adverse terrain.
The prototype first flew on 7 March 1927. Initial tests showed poor control, and the prototype was modified with a much larger tail and horn-balanced ailerons, solving these problems. (It was later discovered that a 2 foot (0.61 m) fuselage section had been omitted from the prototype - as handling was now acceptable, it was not re-instated.) The Wapiti performed well during RAF trials, while using significant amounts of DH.9A components, and was declared the winner of the competition, an initial contract for 25 aircraft being placed in October 1927.
After initial production, the wooden fuselage, tail and wings were replaced by metal structures in the Wapiti II and IIA, and the original long fuselage was eventually re-instated in the Wapiti V and later versions. In 1930, Westland produced an updated version of the Wapiti, the Wapiti VII, which differed so much that it was renamed the Westland Wallace.