Operational experience with the O/100 showed that certain changes were desirable, especially to the fuel system. In the original layout each engine had its own armoured fuel tank contained within the armoured nacelle which housed the engine, restricting the amount of fuel which could be carried. The modified fuel system consisted of two fuselage tanks and two gravity-fed tanks installed in the leading edge of the upper wing's centre-section. Wind-driven pumps supplied fuel direct to the engines, as well as to the gravity-fed tanks. Removal of the fuel tanks from the nacelles allowed them to be shortened and a new inter-plane strut to be fitted immediately aft of each nacelle.

Other improvements included the provision of a compressed-air engine-starting system, with a crank handle for manual start in the event of pressure loss, and changes to the rear gun position and central fin. In this new configuration this variant of the O/100 was redesignated O/400. An initial contract for 100 of these aircraft was awarded to Handley Page in August 1917.

Production deliveries of O/400 began in the spring of 1918, but it was not until 9 August 1918 that No 97 Squadron, which was equipped with these aircraft, joined the Independent Force and began operations. As numbers built up it became possible to launch heavier and more frequent raids: on the night of 14-15 September 1918 an attack by 40 Handley Pages was launched against enemy targets. It was also during September that O/400 began to use newly developed 750kg bombs for the first time.

A single O/400 also played an important role during the final offensive in Palestine, bombing HQs and communications and doing the work of a squadron of smaller machines. The outward flight of this aircraft from Britain to Egypt was important in its own right. After the Armistice the bomber returned to Cairo and from there flew to Delhi and Calcutta .

A total of 700 O/400 were ordered, and about 400 were delivered before the Armistice. In the US 1,500 of these aircraft were ordered from Standard Aircraft Corporation, with power plant comprising two 261kW Liberty 12-N engines, but of this total only 107 were delivered to the US Army Air Service before signature of the Armistice brought contract cancellation. A number of British-built O/400 were delivered post-war to China.

3-View

A three-view drawing (664 x 662)