In 1934Air Ministry Specification P.4/34 was issued which called for a light bomber that could also be deployed in a close-support role as a dive-bomber. Fairey, Gloster and Hawker attempted to fulfil this need and competition was tight to attain the highest performance possible. As the aircraft required only a modest bomb load and with performance being paramount, the Hawker design team chose to focus its efforts on developing an aircraft similar in size to their Hurricane fighter. The Hurricane was then in an advanced design stage and there would be economies of scale if some assemblies were common to both aircraft. This resulted in the Henley, as it was to become known, sharing identical outer wing panel and tailplane jigs with the Hurricane. Both were equipped with the Rolls-Royce Merlin engine as it offered the best power-weight ratio as well as a minimal frontal area. The Henley's cantilever fabric-covered monoplane wing was mid-set, a retractable tail wheel landing gear was selected and accommodation provided for a pilot and observer/air gunner, which differed from the Hurricane's single-seat cockpit. Although construction of a Henley prototype began as early as mid-1935, with priority given to Hurricane development, it was not until 10 March 1937 that, powered by a Merlin "F" engine, it was first flown at Brooklands, shortly after the competing Fairey P.4/34. Subsequently, the aircraft was refitted with light alloy stressed-skin wings and a Merlin I engine and further test flights confirmed the excellence of its performance. It could reach a top speed of. However, at this point the Air Ministry had by this point decided that it no longer required a light bomber. Accordingly, the Henley, which in line with RAF policy had not been fitted with dive brakes; bomb crutches; or specialised bomb sights and thus limited to attack angles of no greater than 70° was relegated to target-towing duty. Henley production was subcontracted to Gloster Aircraft and 200 were ordered into production. The second prototype was fitted with a propeller-driven winch to haul in drogue cable after air-to-air firing sorties. This was first flown on 26 May 1938.
Operational history.
Production Henley III aircraft, entered service with Nos. 1, 5 and 10 Bombing and Gunnery Schools, as well as with the Air Gunnery Schools at Barrow, Millom and Squires Gate. Unfortunately, it was soon discovered that unless the aircraft were restricted to an unrealistically low towing speed of, the rate of engine failures was unacceptably high, attributed to a cooling system matched to the Henley's original missions but inadequate when towing a target at high engine speed but low airspeed. Henleys were relegated to towing larger drogue targets with anti-aircraft co-operation units, predictably proving themselves even less well-suited to this role; the number of engine failures increased and problems with engines were compounded by difficulties releasing drogue targets. Several Henleys were lost after the engine cut out and the drogue could not be released quickly enough. A solution was never found to this problem and in mid-1942, the Henley was withdrawn from service, in favour of adapted Boulton Paul Defiants and specialised Miles Martinet aircraft.
