Second of the four-engine heavy bombers to enter service with the RAF, in November 1940, the Handley Page H.P.57 Halifax was one of the famous triad comprised of the Halifax, Avro Lancaster and Short Stirling which mounted Bomber Command's night-bombing offensive against Germany. In conjunction with the daylight attacks for which the USAAF had accepted responsibility, this round-the-clock battering of German targets reached its peak in 1944, causing almost unbelievable devastation. But although it entered service more than a year ahead of the Lancaster, the Halifax was always somewhat over-shadowed in the bombing role by the achievement of the superb Lancaster. The Halifax, however, scored over the Lancaster in its multi-role capability, for in addition to its deployment as a heavy night-bomber, it was equally at home when employed as an ambulance, freighter, glider tug, personnel transport and maritime reconnaissance aircraft.
The origin of the Halifax stemmed back to an Air Ministry requirement of 1935 for a twin-engine bomber, to which Handley Page submitted a design identified as the H.P.55. This proved to be unsuccessful, but about a year later the Air Ministry issued a new specification, P.13/36, which called for a medium/heavy bomber to be powered by a 24-cylinder engine known as the Vulture which Rolls-Royce then had under development. Handley Page's H.P.56 proposal was selected for prototype construction, but the company had doubts that the Vulture engine would emerge as a reliable production powerplant, and set about the task of redesigning the H.P.56 to take four Rolls-Royce Merlins instead. The overall configuration was not greatly changed, but the H.P.57 design which was submitted to the Air Ministry for approval was for a considerably larger and heavier aeroplane.
On 3rd September 1937 Handley Page was awarded a contract for the manufacture of two prototypes of the H.P.57, with construction beginning in early 1938. When the first of these was nearing completion, it was realized that the company's airfield at Radlett, Hertfordshire, was too restricted for the first flight of such a large aircraft, and it was decided instead to use the nearest non-operational RAF airfield, which was at Bicester in Oxfordshire. Thus, final assembly was carried out at Bicester and it was from there that the first flight was made on 25th October 1939.
As then flown the H.P.57 was a mid-wing cantilever monoplane of all-metal construction, the wing incorporating automatic leading-edge slats, but these were deleted on production aircraft as the Air Ministry required that the wing leading edges should be armoured and provided with barrage balloon cable cutters. The tail unit comprised a large high-mounted tailplane and rudder assembly with twin endplate fins and rudders, and the fuselage was a deep, slab-sided all-metal structure with considerable internal volume; it was this feature which was to provide the later versions with multi-role capability. Accommodation was provided for a crew of seven, including three gunners to man nose, beam and tail positions. Landing gear was of retractable tailwheel type, and the powerplant comprised four Merlin engines. For its primary role, as a bomber, a variety of weapons could be carried in a 22 ft (6.71 m) long bomb bay in the lower fuselage, supplemented by two bomb compartments in the wing centre-section, one on each side of the fuselage.
The second prototype made its first flight on 17th August 1940, followed just under two months later by the first production example, by then designated Halifax Mk I, and this was powered by 1,280 h.p. (954 kW) Rolls-Royce Merlin X engines. Armament of these early production aircraft consisted of two and four 0.303 in (7.7 mm) machine-guns in nose and tail turrets respectively.
Full designation of the first production version was Halifax B.Mk I Series I, and these began to equip the RAF's No. 35 Squadron during November 1940. It was this unit that, in early March 1941, was the first to use the Halifax operationally, in an attack on Le Havre, and a few days later the Halifax became the first of the RAF's four-engine bombers to make a night attack against a German target, when bombs were dropped on Hamburg. The Halifax was used for the first time in a daylight attack against Kiel on 30th June 1941, but it did not take long to discover that the aircraft's defensive armament was inadequate for daylight use, and by the end of 1941 the Halifaxes were used only by night in the bombing role. This resulted in the provision of better armament for later versions.
Early deployment of the Halifax had confirmed that this new four-engine bomber had much to offer, but although contracts for large-scale construction very quickly exceeded the productive capacity of the Handley Page factories at Cricklewood and Radlett, prewar plans had been made for alternative sources of supply. The establishment of four new production lines was made easier by the unit method of construction which had been adopted for the Halifax, and the first of these sub-contract aircraft to fly, on 15th August 1941, came from the English Electric Company, which had earlier been involved in manufacture of Handley Page's Hampden medium bomber. The other three lines were those of Fairey at Stockport, Rootes Securities at Speke, and the London Aircraft Production Group.
From their first introduction into operational service, Halifax bombers were in continuous use by Bomber Command, equipping at their peak usage no fewer than 34 squadrons in the European theatre, and four more in the Middle East. Two flights were in early use in the Far East, and following VE-Day a number of squadrons operating with the Halifax B.Mk VI flew their aircraft out for co-operation with the Allied forces fighting in the Pacific theatre. The Halifax was involved in the first Pathfinder operations in August 1942; was the first RAF aircraft to be equipped with the highly secret H²S blind bombing radar equipment; was involved extensively in daylight attacks on German V-1 sites; and between 1941 and 1945 flew 75,532 sorties during which 227,610 tons of bombs were dropped on European targets.
The Halifax was also operated by nine squadrons of the RAF's Coastal Command for anti-submarine, meteorological and shipping patrols, the aircraft being converted from standard bombers and specially equipped, taking the designations Halifax GR.Mk II, GR.Mk V or GR.Mk VI according to the bomber version from which they were derived. Similarly, RAF Transport Command acquired Halifax C.Mk III, C.Mk VI and C.Mk VII aircraft as casualty, freight and personnel transports. Little known in wartime was the work of Nos 138 and 161 (Special Duties) Squadrons, which had the task of dropping special agents and/or supplies by parachute into enemy territory.
One other vital use of the Halifax was by the Airborne Forces, for under the designations Halifax A.Mk III, A.Mk V and A.Mk VII, equivalent bomber versions were converted to serve for the deployment of paratroops or as glider tugs. The Halifax was, in fact, the only aircraft capable of towing the large General Aircraft Hamilcar glider, a capability first proven in February 1942. Soon after that date the Halifax tug made its operational debut when two Airspeed Horsas were hauled across the North Sea to attack the German heavy water plant in south Norway.
The Halifax Mk I was followed into service by the Halifax B.Mk II Srs I, which introduced a Boulton Paul twin-gun dorsal turret, and an increase of 15 per cent in standard fuel capacity; the powerplant, initially Merlin XXs, was changed later to Merlin 22s of equal power output. These changes, plus others introduced after the prototypes had made their first flights, resulted in a steady increase in gross weight. As there had been no surplus engine power from the outset, the result was that operational performance was being eroded by enhanced capability. This can be accepted during wartime conditions provided the rate of attrition remains fairly constant, but in the case of the Halifax Mk II the dorsal turret represented 'the last straw', and steps were taken immediately to improve the performance of these aircraft.
The resulting Halifax B.Mk II Srs IA (company designation H.P.59) had a performance increase of some 10 per cent in both maximum and cruising speeds, which was achieved by efforts to reduce both weight and drag. The nose turret was deleted, the nose acquiring a streamlined fairing; the dorsal turret was removed. Later production switched to the B.Mk II Series IA, which introduced a Perspex nose fairing and Defiant-type four-gun dorsal turret.
A later change, introduced retrospectively to all aircraft then in service, involved replacement of the triangular fins by larger units of rectangular shape. This came after extensive testing, following some inexplicable losses of fully loaded aircraft, had shown that it was possible for the Halifax to enter an inverted and uncontrollable spin. The last major production version was the Halifax B.Mk III (company designation HP.61), the first of the bombers to introduce Bristol Hercules VI or XVI radial engines, which offered 1,615 h.p. (1204 kW) for take-off.
Although withdrawn from Bomber Command immediately after VJ-Day, the Halifax GR.Mk VI continued to serve with Coastal Command after the war, as did the Halifax A.Mk VII with transport squadrons at home and overseas. Post-war versions included the Halifax C.Mk VIII (company designation H.P.70), which could accommodate an 8,000 lb (3629 kg) capacity detachable pannier beneath the fuselage, and the Halifax A.Mk IX (company designation H.P.71) troop-carrier and supply-dropper for use by airborne forces. When production of these two versions ended, amounting to some 230 aircraft, a total of 6,178 Halifaxes had been built, and examples remained in RAF service until late 1947.
When Transport Command Halifax C.Mk VIIIs became surplus to military requirements, 10 were converted by Short Bros and Harland as 10-seat Halton Mk I (H.P.70) civil transports for service with BOAC. Subsequently about 80 other civil conversions, some to near Halton standard, were carried out by a variety of contractors.

Variants:
Halifax B.Mk I Srs II:
Generally similar to B.Mk I Srs I, but stressed for operation at a higher gross weight.
Halifax B.Mk I Srs III:
Version of B.Mk I Srs I with increased fuel capacity; late production examples introduced Merlin XX engines.
Halifax B.Mk II Srs I (Special):
Redesignation of B.Mk II Srs Is which incorporated in-service modification of as many as possible of the improvements of the B.Mk II Srs IA.
Halifax B.Mk V Srs IA:
As B.Mk II Srs IA, except for introduction of Dowty landing gear; the company designation was H.P.63.
Halifax B.Mk V Srs I (Special):
As B.Mk II Srs I (Special), except for introduction of Dowty landing gear and hydraulics.
Halifax B.Mk VI:
Generally as B.Mk III, but with 1,675 h.p. (1249 kW) Bristol Hercules engines which developed 1,800 h.p. (1342 kW) at 10,000 ft (3050 m); company designation H.P.61.
Halifax B.Mk VII:
As B.Mk VI, but reverting to use of Bristol Hercules XVI radials as a result of shortage of Hercules 100s; company designation H.P.61.