The BHP engine was unreliable and failed to provide the expected power, which gave the DH.9 poorer performance than the aircraft it had been meant to replace.

During June 1917, partially as a result of attacks by German bombers on London, the War Office issued its recommendation to almost double the size of the Royal Flying Corps (RFC) to a total of 200 squadrons.

[1][2] While an order for 700 DH.4s was placed on 28 June 1917, on 23 July 1917, the Air Board were presented with drawings for a modified DH.4 that delivering greater range under a new type number, DH.9.

It used the wings and tail unit of the DH.4 but featured a modified fuselage which moved the pilot closer to the gunner/observer and away from the engine and fuel tank, which facilitated communication and was a more optimal fighting configuration.

[3] The other major change from the DH.4 was the choice of the promising new B.H.P engine, which was predicted to produce 300 hp (220 kW) to provide adequate performance to match enemy fighters.

According to aviation author J.M Bruce, the selection of the DH.9 "seems to have been taken in a spirit of optimism or blind faith, for its chosen engine was, in July 1917, experiencing serious manufacturing difficulties".

As early as November 1917, some officials, such as General Hugh Trenchard, raised repeated objections to the aircraft based upon its disappointing performance; however, in his response, President of the Air Council Sir William Weir started that "it was the choice of having the DH.9 with the B.H.P.

[3] Another alternative powerplant was adopted in the form of the 430 hp (320 kW) Napier Lion engine; one such Lion-engined aircraft achieved a World Altitude Record of 30,500 ft (9,300 m) on 2 January 1919.

[3] The standard flight surfaces were broadly the same, but adopted a highly redesigned fuselage configuration, including the repositioning of the pilot's cockpit to a more rearwards position.

According to the testing squadron, the DH.9 possessed a high level of manoeuvrability, was relatively easy to perform landings aside from a poor view during the approach.

During the war, numerous customisations and improvised improvements were made to the aircraft by the maintenance crews of individual squadrons, often for the purpose of addressing the type's engine performance issues.

[12] After the end of the war, many DH.9s, which had been originally delivered as bombers, were sold; these were often reconfigured to serve in different roles, including passenger and cargo transport, trainer aircraft, and as air ambulances.

[13] To boost the rate of production, quantity orders for the DH.9 were also placed with Alliance, G & J.Weir, Short Brothers, Vulcan, Waring & Gillow and National Aircraft Factories No.

[15] On multiple occasions, less than half of a flight of bombers would reach their intended targets; according to Bruce, the successful missions performed by the DH.9 were frequently a product of "the courage and determination of the pilots and observers that flew them".

Squadrons would often implement their own home-built enhancements to their aircraft, such as enlarged carburetor air intakes and modified fuel mixture controls.

[14] Nevertheless, the type proved capable during some engagements; on 23 August 1918, a DH.9 flown by Lieutenant Arthur Rowe Spurling of 49 Squadron, with his observer, Sergeant Frank Bell, single-handedly attacked thirty Fokker D.VII fighters, downing five of them.

Simpson of 49 Squadron, while flying a bombing mission upon bridges at Falvy and Bethencourt, shot down four hostile fighters while defending against repeated attacks upon the formation.

[19] In another instance, Captain John Stevenson Stubbs achieved 11 aerial victories in a DH.9, including the highly unusual feat of balloon busting while flying the type.

[21] Amongst those aircraft used in Somalia, one DH.9 was modified to function as an air ambulance, being able to load and transport a single stretcher case, which was carried upon the upper longerons directly to the rear of the pilot's cockpit.

[25] Following the end of the conflict, large numbers of surplus DH.9s became available at low prices and the type was widely exported; this included those aircraft that were donated to the Dominions as well as India as part of the Imperial Gift programme.

[38] (Part of Imperial Gift) Of the thousands of DH.9s built, only a few survived to be preserved: In 2000, the remains of three DH.9s were discovered in India and two were retrieved for restoration in the UK.