The one-off Handley Page HP.51 bomber-transport prototype of 1935 (designed against the 1931 Specification C.26/31) was selected for serial production to become the HP.54 "Harrow" heavy bomber. The revised design retained the former's twin-engine arrangement and high-wing monoplane and was flown for the first time on October 10th, 1935. Produced from 1936 to 1937, one hundred examples were ultimately realized across two major variants, Mk.I and Mk. II - these going on to serve both the British Royal Air Force (RAF) and Royal Canadian Air Force for their time in the air.
Design of the series was attributed to G.V. Lachmann (1896-1966) and the aircraft was offered specifically against the newly drawn Specification B.29/35 of 1935 for the dedicated heavy bomber role. Even before the finalized design flew, the RAF - desperate to modernize its attack wings - contracted for 100 of the type.
The end-result was a relatively modern bomber retaining its transport capabilities. The aircraft featured a stepped cockpit aft and above a glazed nose section and, beyond the high-wing monoplane, were twin vertical rudder fins and a fixed, wheeled undercarriage. The crew numbered five and defensive gun positions allowed for up to 4 x 7.7mm Lewis Machine Guns to be fitted in turrets set about the airframe. The single-gunned nose and twin-gunned tail turrets were power-assisted while the single-gunned dorsal turret was manually-trained. Beyond this armament was an inherent capability to carry up to 3,000lb of conventional drop ordnance.
In its original Harrow Mk.I production form (of which 19 were eventually built), the bomber was powered by 2 x Bristol Pegasus X air-cooled radial piston engines of 830 horsepower driving three-bladed propeller units. The follow-on Harrow Mk.II variant, definitively produced in 81 examples, carried twin Bristol Pegasus XX series radials of 925 horsepower.
Structural dimensions included a running length of 82.1 feet, a wingspan of 88.4 feet, and a height of 19.4 feet. Empty weight reached 13,600lb against a gross of 23,000lb. Performance figures included a maximum speed of 200 miles-per-hour, a cruising speed of 165 mph, a service ceiling of 22,800 feet, and a rate-of-climb nearing 710 feet-per-minute.
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1 x 7.7mm Lewis Machine Gun in power-assisted nose turret.
1 x 7.7mm Lewis Machine Gun in manually-trained dorsal turret.
2 x 7.7mm Lewis Machine Guns in power-assisted tail turret.
Up to 3,000lb of conventional drop bombs.
HP.54 "Harrow" - Base Series Designation.
Harrow Mk.I - Initial production model with Bristol Pegasus X series engines of 830 horsepower; 19 examples completed.
Harrow Mk.II - Definitive production model with Bristol Pegasus XX engines of 925 horsepower; 81 examples completed.
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