The Halifax has its origins in the twin-engine HP56 proposal of the late 1930s, which had been produced in response to the British Air Ministry's Specification P.13/36 for a capable medium bomber for "world-wide use". The HP56 was ordered as a backup to the Avro 679, both designs being designed to use the underperforming Rolls-Royce Vulture engine; the Handley Page design was altered at the Ministry to a four-engine arrangement, which was powered by the Rolls-Royce Merlin engine, while the rival Avro 679 was produced as the twin-engine Avro Manchester which, while regarded as unsuccessful mainly due to the Vulture engine, was a direct predecessor of the famed Avro Lancaster. Both the Lancaster and the Halifax would emerge as capable four-engined strategic bombers of which thousands would be manufactured and operated by the RAF and several other services during the War.
On 25 October 1939, the Halifax performed its maiden flight, and entered service with the RAF on 13 November 1940. It quickly became a major component of Bomber Command, performing routine strategic bombing missions against the Axis Powers, many of which were performed at night. Arthur Harris, the Air Officer Commanding-in-Chief of Bomber Command, described the Halifax as being inferior to the rival Lancaster, in part due to its inability to carry larger individual bombs such as the 4,000 pound "Cookie" blast bomb. Nevertheless, production of the bomber continued until April 1945. During their service with Bomber Command, Halifaxes flew a total of 82,773 operations and dropped 224,207 tons of bombs, while 1,833 aircraft were lost. The Halifax was also operated in large numbers by other Allied and Commonwealth nations, such as the Royal Canadian Air Force (RCAF), Royal Australian Air Force (RAAF), Free French Air Force and Polish forces.
Various improved versions of the Halifax were introduced, which incorporated more powerful engines and a revised defensive turret layout and also made it capable of carrying increased payloads. It remained in service until the end of the war, performing a variety of duties in addition to bombing. Additionally, specialised versions of the Halifax were developed for troop-transport and paradrop operations. Following the end of the Second World War, the RAF quickly chose to phase the Halifax out of service, the type having been succeeded in the strategic bombing role by the Avro Lincoln, an advanced derivative of the Lancaster. During the post-war years, the Halifax was operated by the Royal Egyptian Air Force, the French Air Force and the Royal Pakistan Air Force. The type also entered commercial service for a number of years, where it was mainly used as a freighter. A dedicated civil transport variant, the Handley Page Halton, was also developed and entered airline service. 41 civil Halifax freighters were used during the Berlin Airlift. In 1961, the last remaining Halifax bombers were retired from operational use.
In the 1930s, the Royal Air Force (RAF) was primarily interested in twin-engine bombers. These designs put significant demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. Power limitations were so serious that the British invested heavily in the development of huge engines in the 2,000 horsepower (1,500 kW) class in order to improve performance. During the late 1930s, none of these were ready for production. Both the United States and the Soviet Union were pursuing the development of bombers powered by arrangements of four smaller engines, the results of these projects proved to possess favourable characteristics such as excellent range and fair lifting capacity. Accordingly, in 1936, the RAF also decided to investigate the feasibility of the four-engined bomber.
During the mid 1930s, the British Air Ministry formulated and released Specification P.13/36, which sought a twin-engine heavy-medium bomber suitable for "world-wide use". Further requirements of the specification included the use of a mid-mounted cantilever monoplane wing, all-metal construction; the adoption of the in-development Rolls-Royce Vulture engine was also encouraged. In response, Handley Page produced the twin-engine HP56 design to meet Specification P.13/36. Handley Page aircraft designer George Volkert had responsibility for the design.