With the H.P.51 as it's prototype, the H.P.54 Harrow went straight into production after Hitler's 1935 statement that the Luftwaffe was already 30% larger than British intelligence had estimated, even though it was acknowledged to be outdated in terms of front-line service.
The Harrow was to bomber with a secondary role as standby transport. It was planned for quick manufacture, using American automobile manufacturing techniques of component sub-assembly and final completion - allowing HP to gear up to become a major aircraft manufacturer.
The initial Harrows were delivered without turrets, and features like anti-icing and steam-heating were progressively added to production aircraft. Late production aircraft with upgraded engines were designated as Harrow Mk.II's.
Squadron delivery started in January 1937, and they remained in service as night bombers until the end of 1939, when they were replaced by Wellington bombers, and used as transports and as trainers. They did not see active service as bombers.
One interesting role undertaken was that of five aircraft used as night fighters. The aircraft were rigged to carry "Long Aerial Mines" for Operation Mutton. These were explosive charges attached by a 2000ft length of piano wire to a small parachute designed to be dropped above and ahead of an approaching bomber force. The trials were inconclusive, and so the planned Douglas DB-7 Havoc-Pandora aircraft which would have formed a planned defensive force were not acquired.
Three aircraft were transferred to Flight Refuelling Ltd in 1938, and were used for refuelling Empire flying boats on the Britain-Canada route. With the start of hostilities the service was suspended, and the two aircraft in Canada were impressed into the RCAF with one flying and the other being used for spares.