Despite incorporating several modern features, including hydraulically-powered turrets, steam-based cabin heating, and variable-pitch propellers, the performance of the Harrow did not match that of contemporary bombers such as the Armstrong Whitworth Whitley.

While the Fleet Air Arm also placed a separate order for one hundred Harrows, Handley Page did not have the capacity to produce these aircraft.

It participated in several major actions, including an unorthodox defensive role for The Blitz in the winter of 1940–1941 and Operation Market Garden in September 1944.

The aviation author C. H. Barnes observes that, despite rumours to the contrary, the development of the Harrow had nothing to do with Air Ministry Specification B.9/32, for a specialist medium bomber; several features of the aircraft, such as its fixed undercarriage and general construction, were incompatible with the requirements laid out.

[2] Recognising that production of existing bombers, such as the Armstrong Whitworth Whitley, or upcoming projects be reasonably accelerated, the Air Ministry came to recognise that additional types of aircraft would have to be procured if it was to keep pace with political pressures to equip the Royal Air Force, which was undergoing a period of rapid expansion and modernisation, including the growth of its bomber force, as the situation in Europe deteriorated.

Modern construction methods drawn from studies of American automotive factories were adopted, which included a sub-assembly flow system and priority being placed on a high standard of practical training.

[5] The Fleet Air Arm also placed its own order for another hundred but Handley Page found that it lacked the production capacity to supply them.

[7] In March 1937, following the forced landings of seven Heyfords in bad weather, it was ordered that improved navigator positions be adopted on all bombers wherever feasible, leading to urgent modifications being performed on all Harrows.

[9] The majority of Harrows were powered by a pair of Bristol Pegasus XX radial engines, capable of producing up to 925 hp (690 kW).

[6][10] Despite the presence of cabin heating, which used the exhaust head of onboard steam boilers, the Harrow gained a reputation amongst air crews of being a cold and draughty aircraft, which was largely attributed to the design of its turrets.

Perhaps its most prominent use during this brief period was a series of public formation flights performed over various British cities to mark Empire Air Day in May 1938; the Harrow was also put on stand-by during the Munich Crisis of 1938.

[11][16] As a transport, the Harrow was routinely used to convey equipment and personnel between domestic military bases across Britain, as well as to continental airfields prior to the Fall of France.

Meanwhile four Empire flying boats were equipped to receive fuel by this method, and between August and September of 1939, 15 trans-Atlantic crossings were made using this system.

The trials came to an end with the outbreak of World War II, and the two Harrows based at Gander were pressed into service with the Royal Canadian Air Force.