The Bristol Blenheim is a British light bomber aircraft designed and built by the Bristol Aeroplane Company that was used extensively in the first two years of the Second World War. It was originally developed as the civil-orientated Type 142 in response to Lord Rothermere's challenge to produce the fastest commercial aircraft in Europe. First flying in April 1935, the Air Ministry was quickly impressed by its performance and ordered a modified design as the Type 142M to serve in the Royal Air Force (RAF) as a bomber. On 10 March 1937, deliveries of the newly named Blenheim commenced to RAF squadrons.
The Blenheim was one of the first British aircraft to feature an all-metal stressed-skin construction, retractable landing gear, flaps, a powered gun turret and variable-pitch propellers. The design of the Blenheim proved to be readily adaptable, and was modified to serve in roles such as an interim long-range fighter and as a night fighter, pending the availability of a more advanced fighter derivative of the Blenheim, the Beaufighter. A more capable bomber derivative, the Beaufort, was also developed, being both larger and heavier than the Blenheim.
A Canadian-built variant named the Bolingbroke was used as an anti-submarine patrol aircraft and trainer. The Blenheim Mk I outran most biplane fighters in the late 1930s but stood little chance against the German Messerschmitt Bf 109 during daylight operations of the Second World War, though it proved successful as a night fighter. The Mark IV variant was equally unsuccessful in its daylight bombing role, suffering many losses in the early stages of the war. The Blenheim was also used by a wide range of overseas operators, as well being produced under licence in Finland and Yugoslavia.
In early 1934, Lord Rothermere, owner of the Daily Mail newspaper, challenged the British aviation industry to build a high-speed aircraft capable of carrying six passengers and two crew members – he referred to the ambition as seeking "the fastest commercial aeroplane in Europe, if not the world". At the time, German firms were producing a variety of record-breaking high-speed designs, such as the single-engined Heinkel He 70, and Rothermere wanted to recapture the title of fastest civilian aircraft, as well as to purchase such an aircraft himself. Rothermere also intended to encourage businesses and key figures to make greater use of civil aviation, and to demonstrate to the British Air Ministry how their fighter aircraft may not be able to match modern transport aircraft, which may be easily converted to, or used as the basis for, a bomber aircraft.
Since July 1933, Frank Barnwell, Bristol's chief designer, had been working on a small twin-engine low-wing monoplane design, initially intended to be powered by the sleeve-valve Bristol Aquila radial engine, designated as the Type 135. Rothermere became aware of Bristol's proposal and, in response to his inquiry, on 3 March 1934, Barnwell issued him with a quote of the specification and performance statistics of the design, including an estimated top speed of 240 MPH at an altitude of 6,500 feet. By this point, proposed use of the Aquila engine had been shelved in favour of the supercharger-equipped, poppet-valve Bristol Mercury engine. Deeming it suitable for the issued challenge, the design of Type 135 was further adapted to produce the Type 142 in order to meet the requirements outlined by Rothermere. In late March 1934, Rothermere placed an order for a single Type 142 aircraft, under which he paid for half of the estimated £18,500 cost up front and the remainder upon the aircraft's first flight in the following year.
On 12 April 1935, the Type 142, which had been given the name Britain First, conducted its maiden flight from Filton Aerodrome, South Gloucestershire. Flight tests soon proved that the aircraft was in fact faster than any fighter in service with the Royal Air Force (RAF) at the time, having demonstrated a top speed of 307 MPH. Rothermere presented the aircraft to the nation for a formal evaluation at a potential bomber. By June 1935, the Air Ministry had become interested in the project due to its high performance. On 9 July 1935, a design conference was held by Bristol at the ministry's request into the question of converting the Type 142 into a suitable medium bomber.
Based upon talks from the conference, the Air Ministry quickly formalised Specification B.28/35 for prototypes of a bomber version; the Type 142M (M for military). One principal change between the Type 142M bomber and its Type 142 predecessor was the repositioning of the wing from a low-wing to a mid-wing position, which allowed for more internal space within the fuselage underneath the main spar to accommodate a sizable bomb bay. Other modifications included the addition of a bomb-aimer's position and a Browning machine gun gun in the nose along with provisions for a semi-retractable gun turret in the dorsal position.
Blenheim Mk I reconstruction by the Aircraft Restoration Co at Duxford
, for the owners, Blenheim (Duxford) Ltd. The reconstruction is based upon a restored Bolingbroke airframe with restored Blenheim Mk I nose section.
In September 1935, an initial contract for 150 aircraft was placed. The Air Ministry had chosen to order the type directly from the drawing board, having been urgently sought as one piece of a wider and rapid expansion of the RAF. The first aircraft built of this production model, K7033, served as the only prototype; on 25 June 1936, K7033 conducted its first flight from Filton. The service name for the aircraft became Blenheim Mk I after the famous battle during the War of the Spanish Succession. On 10 March 1937, production deliveries to the RAF formally started; 114 Squadron became the first squadron to receive the Blenheim. On 13 January 1938, the Blenheim entered service with No. 30 Squadron, the first overseas squadron to receive the type; in early 1939, the first Blenheims arrived in India.
From July 1936 onwards, various additional orders were placed for the Blenheim Mk I, including multiple orders for the export market. By the end of 1936, 1,568 aircraft were on order. In order to meet the demand, secondary assembly lines were established at Chadderton by Avro and at Speke by Rootes Securities. The aircraft was built under licence by overseas countries, including Finland, who completed a total of 55 aircraft, and Yugoslavia, which completed 16 aircraft with a further 24 in advanced stages of completion when Germany invaded Yugoslavia. Other countries also procured the Blenheim, including Romania, Greece and Turkey. By September 1939, orders for the Blenheim had risen to 2,088 aircraft. Total production of the Blenheim Mk I in England was 1,351 aircraft prior to the end of the production run in 1939; production had been terminated in favour of more advanced variants.
The Blenheim production program saw several shifts in requirements and in capacity. A modified Blenheim design, given the name Bolingbroke, was manufactured under licence in Canada by Fairchild Aircraft. The Bolingbroke, which had been developed in response to Air Ministry Specification G.24/35 to procure a coastal reconnaissance/light bomber as a replacement for the Avro Anson, had substantial improvements that would serve as the basis for improved variants of the Blenheim. According to aviation author James D. Oughton, both the navigator's station and range limitations of the Blenheim Mk I had been subject to considerable criticism, thus an improved model of the aircraft was desired in order to rectify these shortcomings. On 24 September 1937, an experimental Blenheim Mk I, modified with an extended forward fuselage, made its first flight from Filton.
Formal work on an extended-range reconnaissance version started as the Blenheim Mk II, which increased tankage from 278 gal (1,264 L) to 468 gal (2,127 L). Only one Blenheim Mk II was completed as flight tests revealed the increase in speed to be marginal and thus not warrant further development. Another modification resulted in the Blenheim Mk III, which lengthened the nose, and thereby dispensed with the "stepless cockpit" format of the Mk.I in introducing a true windscreen in front of the pilot, to provide more room for the bombardier. This required the nose to be "scooped out" in front of the pilot to maintain visibility during takeoff and landing. However both of these modifications were instead combined, along with a newer version of the Mercury engine with 905 hp (675 kW) and the turret acquired a pair of Brownings instead of the Vickers K; creating the Blenheim Mk IV.
In early 1939, the first batch of Blenheim Mk IVs were accepted into service; these lacked outer fuel tanks but were accepted due to the urgent demand for the type. Early built Blenheim Mk IVs were also equipped with the Mercury VIII engine, most were fitted with the more powerful Mercury XV or Mercury 25 models. Further aircraft deliveries were made to the production standard and were primarily manufactured by Avro and Roots.Production of the Blenheim IV continued through to June 1943, by which point newcomers such as the Blenheim-derived Beaufighter had succeed the type. A total of 3,307 were produced.
Another modification led to a long-range fighter version; the Blenheim Mk IF. For this role, about 200 Blenheims were fitted with a gun pack under the fuselage for four .303 in (7.7 mm) Brownings. Later, the Airborne Intercept (AI) Mk III or IV radar was fitted to some aircraft in use as night fighters; these were the first British fighters to be equipped with radar. The Blenheim had been selected as the first aircraft to be adapted for this role as its fuselage was sufficiently roomy to accommodate the additional crew member and radar apparatus. Their performance was marginal as a fighter, but they served as an interim type pending availability of the more capable Beaufighter derivative. About 60 Mk IVs were also equipped with the gun pack as the Mk IVF and were used by Coastal Command to protect convoys from German long-range bombers.
The last bomber variant was conceived as an armoured ground attack aircraft, with a solid nose containing four more Browning machine guns. Originally known as the Bisley, (after the shooting competitions held at Bisley), the production aircraft were renamed Blenheim Mk V and featured a strengthened structure, pilot armour, interchangeable nose gun pack or bombardier position, and yet another Mercury variant, this time with 950 hp (710 kW). The Mk V was ordered for conventional bombing operations, with the removal of armour and most of the glazed nose section. The Mk V, or Type 160, was used primarily in the Middle East and Far East.
The Blenheim served as the basis for the Beaufort torpedo bomber, which itself led to the Beaufighter, with the lineage performing two complete circles of bomber-to-fighter.