Handley Page lost little time in designing an aircraft to meet an Admiralty specification of December 1914 for a large twin-engine patrol bomber. Identified originally as the Handley Page Type O, it was later designated O/100, the figure 100 indicating its wing span in feet. This was very much larger than anything else built by the Handley Page company up to that point, and when the prototype was completed, it was the largest aeroplane that had then been built in the UK.
The O/100 was a biplane, with folding wings of unequal span and constant-chord. The wings were mounted on a square-section cross-braced fuselage that terminated in a biplane tail unit. The tailskid landing gear had twin wheels on each main unit and the two 266 hp Rolls-Royce Eagle II engines, were housed in armoured nacelles, mounted between the wings just outboard of the fuselage. Accommodation in the first prototype (serial number 1455) was in a enclosed glazed cockpit, with the floor and sides of the cockpit being protected by armour plate. The O/100 was first flown on 7th December 1915, and was found to be inadequate in performance. The second prototype had a revised open cockpit for a crew of two (with provision for a gunner's position forward), new radiators were introduced for the water-cooled engines, and most of the armour plating for the cockpit and engines was removed. When the machine was first tested in April 1916 there was a marked improvement in performance, to an extent that in early May it was flown with 20 Handley Page employees aboard to a height of just over 7,000 ft.
In August 1916, was the formation of the first 'Handley Page Squadron', and this unit became operational in France later on that same year. Its first recorded bombing attack was made on the night of 16th-17th March 1917 against an enemy-held railway junction. In addition to their use as night bombers on the Western Front, O/100s also equipped the first bomber squadron of the RAF's Independent Force following its establishment on 5th June 1918.
There was a total of 46 O/100s produced.
In the 1924 system the O/100 became the H.P.11.