Back in July 1934 the British Air Ministry issued a specification, the B.3/34 looking for a heavy night bomber with troop transport capabilities that could replace the Handley Page Heyford Biplane bomber that was in service.
John Lloyd, Armstrong Whitworth's chief designer, answered the specification with a design called the A.W.38, which later was given the name of 'Whitley' after the location of Armstrong Whitworth's main factory. This design, was a development of their previous design, the A.W.23 for a bomber/transport aircraft that had lost to the Bristol Bombay in the earlier C.26/31 specification.
The Armstrong Siddeley Tiger IX radial engine was selected for the Whitley, as it was capable of generating up to 795 hp. One of the most innovative features of the Whitley was the adoption of a three-bladed two-position variable pitch propeller built by De Havilland, as the Whitley was the first aircraft to fly in such configuration. Considering that Lloyd was unfamiliar with the usage of flaps on large heavy monoplanes, they were initially omitted from the design. So, in order to compensate, the mid-set wings were set at a high angle of incidence (8.5º) to yield a good take-off and landing performance. Although flaps were introduced later in the design stage, the wing remained unaltered and, therefore, the Whitley flew with a pronounced nose-down attitude when flown with the wings in the cruising position, resulting in considerable drag.
The Whitley also holds the distinction of being the first aircraft with a semi monocoque fuselage, which was built using a slab-sided structure to ease production, to serve with the RAF. This method replaced the tubular construction method which was traditionally employed by Armstrong Whitworth. Instead, the frame was constructed from light-alloy rolled sections, pressings and corrugated sheets. This, according to some author, was a huge step forward as it allowed the Whitley to survive severe damage on operations.
Given the urgency to replace the old Heyford biplane, in June 1935 a verbal agreement was reached to produce an initial batch of 80, forty of them being Whitley Mk.I and the other forties being more advanced Mk.II. Production was initially overtaken at three factories in Coventry with assembly works being made at Baginton Aerodrome. During 1935 and 1936 various contracts were placed for the type, with the Whitley being ordered 'off the drawing board', before the first flights of the prototype.
On 17th March 1936 the first prototype flew for the first time in Baginton Aerodrome, in Warwickshire. It was piloted by Alan Campbell-Orde, who was Armstrong Whitworth's chief test pilot. This prototype was powered by two Armstrong Siddeley Tiger IX radial engines which yielded 795 hp of power. The second prototype, however, was powered by two Armstrong Siddeley Tiger XI engines, which were a medium-supercharged variant of the IX version.
The prototypes barely differed from the production version and a total of 34 Whitley Mk.I were completed.
Sources:
1. https://en.wikipedia.org/wiki/Armstrong_Whitworth_Whitley
2. Hall Park Books - Warpaint 21 - Armstrong Whitworth Whitley
3. https://www.valka.cz/Armstrong-Whitworth-A-W-38-Whitley-Mk-I-t6567
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