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The Halifax was the second of the famous trio of four engined heavy bombers to see service with the RAF during the Second World War. In partnership with the Avro Lancaster the Halifax brought the war home to Germany in 1944. The Halifax had a lot in common with its more famous colleague. Both were originally designed as two engined bombers using the Rolls Royce Vulture engine. The failure of this engine forced both Handley Page and Avro to adapt their design to use four of the existing Merlin engines.
The Halifax was the final result of a design process that began in 1935, in response to an Air Ministry Specification that called for a twin engined heavy bomber. Handley Page submitted a design, internally designated the HP 55, but lost out to the Vickers Warwick (which itself never saw service as a bomber).
The next year the Air Ministry issued two more bomber specifications, one for a four engined bomber, which produced the Short Stirling, and one (P.13/36) for a two engined bomber, which somewhat ironically resulted in both the Avro Lancaster and Handley Page Halifax four engined heavy bombers.
P.13/36 was based around the Rolls Royce Vulture engine. The Vulture was a complex engine, expected to produce 1,700 hp. The Vulture was made up of two Kestrel engines combining to power a single propeller. It was a potentially powerful design, but was never mechanically reliable, and would ruin the Avro Manchester. Amongst the designs produced in responce to P.13/36 was the Boulton Paul P.91.
Handley Page submitted the HP 56 design. Apart from the engines this was very similar to the eventual Halifax. However, before it could reach prototype stage the Vulture project began to run into problems. Avro continued working on the two engined design, while Handley Page modified their design to use four engines. Although Handley Page wanted to use radial engines, Air Ministry tests suggested that Rolls Royce Merlin engines would give the bomber longer range, and so the new HP 57 design used the reliable established engine.
Work on the new prototype began early in 1938. The first (unarmed) prototype was completed on 2 September 1939, and first flew on 25 October. By this time war had broken out, and so the first flight took place at RAF Bicester (Oxfordshire), a base considered obscure enough not to be the target of Luftwaffe raids. The first armed prototype first flew on 18 August 1940, only two months before the first production aircraft.
The Halifax had been designed to be easy to produce in parts. At the peak of Halifax production six separate production lines, involving some 41 companies in the Halifax Group were involved in producing the aircraft. In all 6,174 Halifax bombers were produced, third behind the Avro Lancaster (7,377) and Vickers Wellington (11,416). The Halifax was the second most important Bomber Command aircraft, behind the Lancaster, dropping over 200,000 tons of bombs, more than every other type of bomber combined!