With German bombing of London in July 1917, HP was asked to produce a bomber capable of carrying a load double that of the O/400. Initially to be fitted with 600hp Condor engines, the V/1200 design included a rear defensive gun.
With the Condor being not available, the design was reworked to take 4 smaller Eagle engines. Using the technical information gained from the earlier 4-engine O/400, this major modification progressed quickly and necessitated changing the straight wing to a swept-back wing.
The engines were uncowled for the first prototype, but an airship-style armoured cowling was trialled on the second prototype, as was a streamlined rear fairing. Due to the performance loses, both feature were not continued into development. However rudders were a problem and were eventually increased in size for production aircraft.
Deliveries of the V/1500 was delayed due to modifications on the production line, and only 3 aircraft were deployed to France by early November 1918. A first bombing raid to Berlin was delayed due to bad weather on the night of 9/10 November, and the raid then planned for 11/12 November - being cancelled with the Armistice.
The V/1500 was not flown in combat in WW1, and with the end of the war this massive aircraft had no place in the new world and so the construction contracts were cancelled. However one aircraft conducted a long-range flight to India, and while there intervened in the 1919 Afghan rebellion, bombing the arsenal in Kabul - the only warlike action conducted by a V/1500.
An attempt was made at the first crossing of the Atlanic in a V/1500, but due to unserviceabilities Alcock and Brown famously flew their Vickers Vimy first. The V/1500 did get to America, and conducted a series of commercial demonstration flights there.