Shortly after the outbreak of the First World War, Frederick Handley-Page foresaw the need for a long range bomber and a formal specification was issued in December 1941 for four prototypes based on his proposals. The first prototype was delivered 12 months later, making its maiden flight on 15th December and a contract for production aircraft followed. Powered by 2 x Rolls Royce Eagle engines, each of 250 hp, a total of 46 such aircraft were built. Known as the Handley Page O/100, it was limited in speed, however, and soon gave way to the improved O/400, the most significant difference between the two being more powerful Eagle VIII engines of 360 hp.
The Handley-Page O/400 also had a more aerodynamic profile which, in association with the up-rated engines, produced an improvement in maximum speed and altitude together with an increased bomb and fuel load. Carrying a crew of 4/5, its armament comprised 5 x .303in Lewis guns (2 on a scarf ring mounting in the nose, 2 in a mid-upper fuselage position and one at the ventral hatch) and up to 2,000lb of bombs could be carried. The first aircraft was tested with a variety of engines in 1917 and production began at the Royal Aircraft factory in the same year with machines entering RAF service in April 1918. The O/400 was used in support of ground forces on the Western Front and for strategic bombing when, capable of carrying the new 1,650lb bomb, it was deployed in force with up to 40 aircraft per raid. It was the principal heavy bomber used by the RAF at the end of the war and continued in service until being replaced by the Vickers Vimy towards the end of 1919.
A total of 10 RAF squadrons were equipped with the Handley-Page O/400. A single machine was operated by the Australian Flying Corps in the Middle East Air Force (in support of Lawrence of Arabia amongst other things) and, post-war, 6 were sold as transports to China with India and Poland taking one each. A total of 554 were built.