As early as December 1914 during the First World War the Royal Navy's Director of the Air Department, Captain Murray Sueter requested "a bloody paralyser" of an aircraft from Frederick Handley Page for long-range bombing. The phrase had originated from Commander Charles Rumney Samson who had returned from the front.
Coastal patrol adaptations to be developed from the unbuilt Handley Page L/200 and internally designated M/200 and MS/200 for their 200 hp/150 kW engines were initially discussed but Sueter's technical advisor Harris Booth favoured a large seaplane for coastal patrol and dockyard defence that would also be capable of bombing the German High Seas Fleet at its base in Kiel: a prototype (the AD Seaplane Type 1000) had already been commissioned from J Samuel White & Co. of Cowes.
Handley Page responded to the Navy's requirements with a biplane having a wingspan of 100 ft/30 m (the original source of the O/100 designation). The first prototype flew on 7 December 1915 and featured a glazed cockpit and armour sufficient to protect from rifle fire around the crew compartment and engines. The aircraft proved somewhat underpowered, so the glazing and armour were deleted on the second prototype that flew the following April and formed the basis for series production of the machine. A total of 46 of the O/100s were built.
The success of the type prompted the development of an uprated version with more powerful engines and other refinements, designated the O/400. First flying in 1918, over 400 were supplied before the Armistice. Another 107 were licence-built in the USA by the Standard Aircraft Corporation (out of a total order of 1,500 by the air corps). Forty-six out of an order for 50 were built by Clayton & Shuttleworth in Lincoln.