The Bristol Blenheim was developed from the private-venture Bristol 142, and the short-nosed Mk 1 entered service as a light bomber in March 1937, although some were completed as fighters. The Blenheim was an effective bomber, but lacking adequate defensive armament and armor, it was vulnerable to fighter attack. The most numerous versions were the long-nosed Mk IV and V, but their performance suffered from significant weight growth, the Mark V in particular suffering heavy losses. The Blenheim nevertheless filled an important capability gap in time of need, and it was exported to Finland, Romania, Turkey, and Yugoslavia. A total of 5,213 Blenheims of all versions were built.
The Vickers Wellington entered service with the RAF late in 1938 and (with the Whitley and Hampden) bore the brunt of the RAF bomber offensive for the first two years of the war. Its light but strong geodetic structure enabled it to carry a respectable bomb load, and it could withstand a significant amount of battle damage. The Wellington was one of the first monoplane bombers to be fitted with power turrets, but (in common with all early World War II bombers) it was vulnerable to fighter attack when flown unescorted in daylight. The Wellington was mainly employed as a medium bomber, although some were used for maritime reconnaissance, torpedo-bombing, minelaying, and transport duties. Wellingtons were in production throughout the war, 11,461 being built up to October 1945.
The Handley Page Hampden entered RAF service late in 1938. Of imaginative design, it delivered a reasonable performance on only average engine power, but the cramped fuselage caused crew fatigue, and the defensive field of fire was very limited. Hampdens were used as medium bombers and minelayers until late 1942, and they served as torpedo-bombers and maritime reconnaissance aircraft until the latter part of 1943. A total of 1,430 Hampdens and variants were built.
The Short Stirling, the first of the RAF’s four-engine “heavies” to see combat, entered service in late 1940. It was built to specification B.12/36, which unfortunately specified that the wingspan should be less than 100 ft to fit in a standard hangar; this compromised the aircraft’s altitude capability to the extent that attacks on Italy required British pilots to fly through the Alps rather than over them. However, the Stirling was outstandingly maneuverable for such a large aircraft. It was used as a bomber, minelayer, glider tug/transport, and (with 100 Group) an electronic countermeasures aircraft. A total of 2,381 Stirlings were built.
The Handley Page Halifax I entered service early in 1941 and was found to be a good bomber, but it lacked adequate defensive armament. The Halifax B.II had a dorsal gun turret but suffered from weight growth and a tendency to spin when fully loaded. Later B.IIs underwent a weight- and drag-reduction program and had larger fins fitted to correct these faults. The B.III version was the most numerous, using more powerful Bristol Hercules engines in place of the Merlins. Although the Halifax’s main role was as a bomber, it was also employed as a transport, glider tug, and maritime reconnaissance aircraft. A total of 6,176 Halifaxes were built.
The Avro Lancaster was a successful development of the Rolls-Royce Vulture-powered Manchester, entering operational service with the RAF in early 1942. The Lancaster remained in service until the end of the war and rapidly became the primary strategic bomber for the RAF. It lost fewer aircraft per ton of bombs dropped than either the Halifax or Stirling. The Lancaster had a large bomb bay and was designed to take 4,000 lb bombs; successive modifications enabled it to carry 8,000 lb and 12,000 lb weapons, and the B.I (special) carried a single 22,000 lb “Grand Slam” armor-piercing bomb. The Lancaster participated in several special operations, including the Dambusters raid in May 1943, when specially adapted Lancasters of 617 Squadron attacked dams in the Rhine valley using a skipping bomb designed by Barnes Wallis. A total of 7,366 Lancasters were built.
The De Havilland Mosquito was constructed largely from a plywood/balsa sandwich and was designed to be fast enough to outrun enemy fighters. It had excellent handling characteristics. It began operations with the RAF in the bomber role early in 1942 and quickly demonstrated that it could carry out extremely accurate attacks, including the daring low-level attack on the Gestapo headquarters in Oslo, Norway, in late 1942. Mosquitoes originally equipped the RAF’s pathfinder force, and they were able to roam across Germany largely unmolested. Operationally, the Mosquito had by far the lowest loss rate of any aircraft in Bomber Command (about 0.6 percent), as its speed enabled it to avoid most interception and its structure tended to absorb cannon hits. A total of 6,439 Mosquitoes of all marks were built.