Specification B.3/34 ushered in the era of the monoplane bomber for the Royal Air Force by asking, as it did, for modern twin-engine designs to replace the Handley Page Heyford and the lumbering Vickers Virginia.
Two companies were awarded contracts to the specification, one being Armstrong Whitworth who submitted the Whitley and the other Handley Page, whose Handley Page H.P.54 was somewhat less original in concept, featuring a high wing and fixed landing gear. It should be acknowledged, however, that although the two designs were to the same basic specification the Handley Page H.P.54, subsequently named Harrow, was intended for initial use as an interim bomber/trainer and later, when more advanced bombers were in quantity production, as a transport aircraft. One hundred Harrows were ordered to a new specification, B.29/35, before the prototype flew on 10th October 1936. This new aircraft was based largely on the H.P.51 prototype troop-carrier, which had flown in May of the previous year.
Handley Page had initiated a new method of production for the Harrow, which enabled components to be manufactured by small firms under sub-contract, offering advantages both in construction and repair. The first 39 production aircraft were designated Harrow Mk I with 850 h.p. (634 kW) Bristol Pegasus X engines, which conferred a top speed of 190 mph (306 km/h), but the following 61 aircraft were Harrow Mk II aircraft with Pegasus XX engines of 925 h.p. (690 kW) giving an extra 10 mph (16 km/h). Power-operated gun turrets in the nose, tail and mid-upper positions were an advance over then current service types; although the Harrow Mk Is did not have them when delivered, they were fitted later.
No. 214 Squadron at Feltwell was the first unit to receive Harrows, in January 1937, when the type began to replace Virginias, and by the end of that year four other squadrons had re-equipped with the new bomber: Nos 37 (Feltwell), 75 (Driffield), 115 (Marham) and 215 (Driffield). No. 115 had been disbanded in 1919, and was re-formed in June 1937 to receive the Harrow, while No. 37 was also reformed in April of that year from a nucleus of No. 214 Squadron.
Harrow production terminated with the 100th example in December 1937, but aircraft remained in service until the late stages of World War 2.
A novel use of the Harrow was as an aerial minelayer when, in October 1940, No. 420 Flight was formed at Middle Wallop to carry out experiments under the codename 'Pandora'. These aircraft carried 'Long Aerial Mines' (LAMs), which consisted of many small explosive charges suspended from parachutes with a 2,000 ft (610 m) length of piano wire trailing below. It was intended they should be launched in the path of a bomber stream, and if one of these aircraft flew into the wires it was expected to release one or more of the charges, which then slid down the wires to explode on contacting the enemy bomber. Three months of trial proved the idea to be impractical, although four or five 'kills' were achieved.
No. 271 Squadron formed at Doncaster on 1st May 1940 to operate m the transport role, equipped with Harrows, Bristol Bombays and some impressed civil aircraft, and although most of the other types had been replaced by 1944 a flight of Harrows was retained. These aircraft found employment in support of Allied forces operating in North West Europe, two of them evacuating casualties from the Arnhem operation in September 1944, and they remained in service until the flight re-equipped with Douglas Dakotas in May 1945.