||Less well-known than the Avro Lancaster, the Halifax was the other mainstay of Bomber Command operations between 1943 and 1945. Originally designed as a twin-engine bomber, the prototype Halifax eventually flew for the first time in October 1939 with four engines. The first production model was delivered in October 1940 and eventually two distinct streams of Halifax were produced: those powered by Rolls Royce Merlin engines (Marks I, II, and V) and those powered by Bristol Hercules engines (Marks III and VI - IX). The Halifax was widely known as the "Halibag" by its crews.
Early Merlin-engine Halifaxes had a relatively low operating ceiling that meant they suffered heavily from ground fire. In 1943 this led to the Halifax being restricted to attacking less well defended targets. Attempts to improve the Halifax by streamlining the fuselage, engines, and other components were not entirely satisfactory, so it was decided to install more powerful Bristol Hercules engines. The first operational flight of a Bristol Hercules-engine Halifax occurred in October 1943 with 466 Squadron RAAF. Eventually, Hercules-engine variants accounted for over half of the 6,177 Halifaxes produced during the war. The Halifax was the only British heavy bomber that permanently operated from the Middle East, and it was also widely converted to act as a transport, glider tug, paratroop carrier, and electronic counter-measures aircraft. Production of the Halifax finally ceased in November 1946.