The Lancaster has its origins in the twin-engine Avro Manchester which had been developed during the late 1930s in response to the Air Ministry Specification P.13/36 for a capable medium bomber for "world-wide use". Originally developed as an evolution of the Manchester (which had proved troublesome in service and was retired in 1942), the Lancaster was designed by Roy Chadwick and powered by four Rolls-Royce Merlins and in one version, Bristol Hercules engines. It first saw service with RAF Bomber Command in 1942 and as the strategic bombing offensive over Europe gathered momentum, it was the main aircraft for the night-time bombing campaigns that followed. As increasing numbers of the type were produced, it became the principal heavy bomber used by the RAF, the RCAF and squadrons from other Commonwealth and European countries serving within the RAF, overshadowing contemporaries such as the Halifax and Stirling.
A long, unobstructed bomb bay meant that the Lancaster could take the largest bombs used by the RAF, including the 4,000 lb (1,800 kg), 8,000 lb (3,600 kg) and 12,000 lb (5,400 kg) blockbusters, loads often supplemented with smaller bombs or incendiaries. The "Lanc", as it was affectionately known, became one of the more famous and most successful of the Second World War night bombers, "delivering 608,612 long tons of bombs in 156,000 sorties". The versatility of the Lancaster was such that it was chosen to equip 617 Squadron and was modified to carry the Upkeep "Bouncing bomb" designed by Barnes Wallis for Operation Chastise, the attack on German Ruhr valley dams. Although the Lancaster was primarily a night bomber, it excelled in many other roles, including daylight precision bombing, for which some Lancasters were adapted to carry the 12,000 lb (5,400 kg) Tallboy and then the 22,000 lb (10,000 kg) Grand Slam earthquake bombs (also designed by Wallis). This was the largest payload of any bomber in the war.
In 1943, a Lancaster was converted to become an engine test bed for the Metropolitan-Vickers F.2 turbojet. Lancasters were later used to test other engines, including the Armstrong Siddeley Mamba and Rolls-Royce Dart turboprops and the Avro Canada Orenda and STAL Dovern turbojets. Postwar, the Lancaster was supplanted as the main strategic bomber of the RAF by the Avro Lincoln, a larger version of the Lancaster. The Lancaster took on the role of long range anti-submarine patrol aircraft (later supplanted by the Avro Shackleton) and air-sea rescue. It was also used for photo-reconnaissance and aerial mapping, as a flying tanker for aerial refuelling and as the Avro Lancastrian, a long-range, high-speed, transatlantic passenger and postal delivery airliner. In March 1946, a Lancastrian of BSAA flew the first scheduled flight from the new London Heathrow Airport. In 1963, the last remaining Lancasters were retired by the RCAF.
In the 1930s, the Royal Air Force (RAF) was primarily interested in twin-engine bombers. These designs put limited 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.
The origins of the Lancaster stem from a twin-engined bomber design that had been submitted in response to Specification P.13/36, which had been formulated and released by the British Air Ministry during the mid 1930s. This specification had sought a new generation of twin-engined medium bombers suitable for "worldwide 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". Various candidates were submitted for the specification, such as Fairey, Boulton Paul, Handley Page and Shorts; all submissions were designed around two-engine configurations, using the Rolls-Royce Vulture, Napier Sabre, Fairey P.24 or Bristol Hercules engines. The majority of these engines were under development at this point; while four-engined bomber designs were considered for specification B.12/36 for a heavy bomber, wings which mounted two pairs of engines were still in the experimental stage and required testing at the Royal Aircraft Establishment (RAE), the resulting increase in overall weight of adopting a stronger wing also necessitated further strengthening of the overall aircraft structure.
In response, British aviation company Avro decided to submit their own design, designated the Avro 679, to meet Specification P.13/36. In February 1937, following consideration of the designs by the Air Ministry, Avro's design submission was selected along with Handley Page's bid being chosen as "second string". Accordingly, during April 1937, a pair of prototypes of both designs were ordered. The resulting aircraft, named the Manchester, entered RAF service in November 1940. Although considered to be a capable aircraft in most areas, the Manchester proved to be underpowered and troubled by the unreliability of the Vulture engine. As a result, only 200 Manchesters were constructed and the type was quickly withdrawn from service in 1942.
As early as mid-1940, Avro's chief design engineer, Roy Chadwick, had been working on an improved Manchester design. This redesign was powered by four of the more reliable but less powerful Rolls-Royce Merlin engines, specifically adopted the form of the Merlin "Power Plant" installations which had been developed by Rolls-Royce for the earlier Beaufighter II, installed on a larger wing. Initially, the improved aircraft was designated as the Type 683 Manchester III, it was subsequently renamed as the Lancaster. The prototype aircraft BT308 was assembled by the Avro experimental flight department at Ringway Airport, Manchester; the prototype was constructed from a production Manchester airframe, which was combined with a new centre section designed to accommodate the additional engines. On 9 January 1941, test pilot H. A. "Bill" Thorn performed the prototype's maiden flight at RAF Ringway, Cheshire.