The early 1930s finally saw acknowledgement of the superior qualities and efficiency of the monoplane wing compared with the biplane, particularly for larger weight carrying machines and all the designs pro posed by manufacturers tendering to meet the C26/31 Specification for a bomber transport were monoplanes. Vickers pro posed but did not build their Type 231, Armstrong-Whitworth built the AW23 and Bristol the prototype Type 130 which won a production order and went into service with the RAF as the Bombay.
The Handley Page machine was the HP51, a high-wing monoplane powered by two Armstrong Siddeley Tiger engines of 700 hp each driving two-blade propellers. The fuselage was taken from the HP43 bomber transport of 1932 and extended by 31 inches but the biplane wing assembly was discarded and replaced by a clean, sharply tapered wing very similar in proportions to that used on the HP47, with the Tigers fully enclosed in neat nacelles. Leading edge slats and slotted flaps were fitted to the wing and the fixed, spatted undercarriage was braced to the engine nacelles and fuselage. A new tail section very like that of the Heyford was fitted and of generally similar appearance to that of the HP43 but of greater span, and the fins and rudders were smaller.
Retaining the serial number J9833 that went with the fuselage, the HP51 was flown from Radlett on 8th May 1935, by Major Cordes. The defensive armament continued to be Lewis guns mounted on Scarff-rings in the open nose and tail positions while the bomb load was carried externally on racks under the fuselage. The aircraft was able to carry 30 troops, 10 stretcher cases or 4,000 lb of bombs on the external racks.
Later in its life the Tiger engines of the HP51 were replaced by 750 hp Bristol Pegasus 3M3 engines driving four-blade propellers, and the fins and rudders were replaced by new taller units with a smooth curved top to the rudder. In this form the aircraft bore a marked resemblance to the HP54 Harrow of late 1936. Later still, in 1938, the HP51 was used by Flight Refuelling Ltd at Ford, Sussex, for trials of the system to be used to air refuel the Short C-class flying boats on the transatlantic mail service in 1939.