The Bristol Blenheim is a British light bomber designed and built by the Bristol Aeroplane Company, which was used extensively in the first two years of the Second World War, with examples still being used as trainers until the end of the war. Development began with the Type 142, a civil airliner, after a challenge from the newspaper proprietor Lord Rothermere to produce the fastest commercial aircraft in Europe. The Type 142 first flew in April 1935, and the Air Ministry ordered a bomber development as the Type 142M for the Royal Air Force (RAF).
Deliveries of the Blenheim to RAF squadrons commenced on 10 March 1937. In service the Type 142M became the Blenheim Mk.I which would be developed into the long-nosed Type 149, the Blenheim Mk.IV, while in Canada Fairchild Canada built the Type 149 under licence as the Bolingbroke. The Type 160 Bisley was also developed from the Blenheim.
Both versions were converted into heavy night fighters by the addition of a gun pack with four Browning machine guns mounted under the fuselage. The Mk.IV was also used as a maritime patrol aircraft and both versions were also used as bombing and gunnery trainers once they had become obsolete as combat aircraft.
The Blenheim was one of the first British aircraft with an all-metal stressed-skin construction, retractable landing gear, flaps, a powered gun turret and variable-pitch propellers. The Mk.I was faster than the RAF's biplane fighters in the late 1930s but advances soon left it vulnerable if flown in the day, though it proved successful as a night fighter. Both Blenheim types were used by foreign operators and examples were licence built in Canada, Yugoslavia and Finland.
Development Origins In 1933 Frank Barnwell, Bristol's chief designer, went to the United States to collect information on their latest twin-engined, low-wing monoplane airliners. When he returned he discussed one of them, the Lockheed Model 12 Electra Junior, with Roy Fedden and prepared a comparable design using Fedden's Bristol Aquila engine which produced , the same power as in the Electra. Sir Archibald Russell described Barnwell's design as "close to being a replica of the Electra".A Span Of Wings, An Autobiography Sir Archibald Russell 1992, , p.39 It was designated Type 135.
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 which he referred to as seeking "the fastest commercial aeroplane in Europe, if not the world". German firms were producing record-breaking high-speed designs, such as the Heinkel He 70, and Rothermere wanted the prestige of having the fastest civilian aircraft. 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 fighters may not be able to match modern transport aircraft, which may be easily converted to, or used as the basis for, bomber aircraft.Moyes 1966, p. 3.
Rothermere became aware of Bristol's Type 135 proposal and on 3 March 1934, Barnwell issued him with a quote of the specification and performance statistics, including an estimated top speed of at . The Aquila engine had been shelved in favour of the supercharger-equipped, poppet-valve Bristol Mercury engine. The Type 135 was further adapted to produce the Type 142 to meet the requirements outlined by Rothermere. In late March 1934, Rothermere ordered one Type 142 aircraft, which he paid for half of the estimated £18,500 cost up front and the remainder following its first flight in the following year.
On 12 April 1935, the Type 142, named Britain First, conducted its first flight from Filton Aerodrome, South Gloucestershire.Barnes 1964, p.258. Flight tests soon proved that the aircraft was faster than the fighters in service with the Royal Air Force (RAF), having a top speed of .Barnes 1964, p. 259.Moyes 1966, pp. 3–4. Rothermere presented the aircraft to the nation for a formal evaluation as a bomber.Moyes 1966, p. 4. In June 1935, the Air Ministry showed interest due to its performance. On 9 July 1935, a design conference was held by Bristol at the ministry's request to convert the Type 142 into a bomber.
The Air Ministry formalised Specification B.28/35 for prototypes of a bomber version which was designated the Type 142M (M for military) by Bristol. A significant change between the Type 142M bomber and its Type 142 predecessor was the raising of the wing from a low-wing to a mid-wing position, which allowed for enough space under the main spar to accommodate a bomb bay. Other modifications included the addition of a bomb-aimer's position and a Browning machine gun in the port wing, along with provisions for a semi-retractable dorsal gun turret.
Production
In September 1935, the Air Ministry placed a contract for 150 aircraft directly from the drawing board, in a wider part of the rapid expansion of the RAF. The first example, K7033, served as the sole prototype and on 25 June 1936, it made its first flight from Filton.Mason 1994, p. 269. The service name for the aircraft became Blenheim Mk.I after the War of the Spanish Succession's Battle of Blenheim. On 10 March 1937, deliveries to the RAF started and 114 Squadron became the first to receive the Blenheim. On 13 January 1938, the Blenheim entered service with No. 30 Squadron, the first overseas squadron and in early 1939, the first Blenheims arrived in India.Moyes 1966, p. 6.
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.Oughton 1971, p. 159. 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 foreign 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.Moyes 1966, p. 11. Other countries also procured the Blenheim, including Romania, Greece and Turkey.Moyes 1966, pp. 11–12. 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.
Further development The Blenheim production programme 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.Oughton 1971, pp. 157, 176. The Bolingbroke, which had been developed in response to Air Ministry Specification G.24/35 to procure a coastal reconnaissance bomber as a replacement for the Avro Anson, had substantial improvements that would serve as the basis for improved variants of the Blenheim.Oughton 1971, p. 157. Both the navigator's station and range limitations of the Blenheim Mk.I had been subject to considerable criticism, prompting the development of an improved model to rectify the shortcomings. On 24 September 1937, an experimental Blenheim Mk.I, modified with an extended forward fuselage beyond its original stepless cockpit, smooth-fronted nose enclosure, made its first flight from Filton.
Formal work on an extended-range reconnaissance version started as the Blenheim Mk.II, which increased tankage from . Only one Blenheim Mk.II was completed, as flight tests revealed the increase in speed to be marginal and not warranting further development. Another modification resulted in the Blenheim Mk.III, which lengthened the nose, dispensing with the "stepless cockpit" format of the Mk.I, introducing a true windscreen in front of the pilot, to provide more room for the bomb aimer. This required the nose to be "scooped out" in front of the pilot to maintain visibility during takeoff and landing. Both modifications were combined, along with a newer version of the Mercury engine with . The turret acquired a pair of Brownings in place of the original single Vickers K gun, creating the Blenheim Mk.IV.Oughton 1971, pp. 157–158.
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 Blenheim Mk.IVs were also equipped with the Mercury VIII engine, most were fitted with the more powerful Mercury XV or Mercury 25 models.Oughton 1971, pp. 158–159. Further aircraft deliveries were made to the production standard and were primarily manufactured by Avro and Rootes.Oughton 1971, p. 158. Production of the Blenheim IV continued until June 1943, when newcomers such as the Beaufort-derived Beaufighter had succeeded the type. A total of 3,307 were produced.
A long-range fighter version, the Blenheim Mk.IF, was also developed. For this role, about 200 Blenheims were fitted with a gun pack under the fuselage for four 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 Ranges), the production aircraft were renamed Blenheim Mk.V and featured a strengthened structure, pilot armour, interchangeable nose gun pack or bomb-aimer position and another Mercury variant with . The Mk.V was ordered for conventional bombing operations, with the removal of armour and most of the glazed nose section. The Mk.V (Type 160) was used primarily in the Middle East and Far East. The Blenheim served as the basis for the Beaufort torpedo bomber, which led to the Beaufighter, with the lineage performing two evolutions of bomber-to-fighter.
Design
The Bristol Blenheim was a twin-engine high performance all-metal medium bomber aircraft, powered by a pair of Bristol Mercury VIII air-cooled radial engines, each capable of . Each engine drove a three-bladed controllable-pitch propeller, and were equipped with both hand-based and electric engine starters. To ease maintenance, the engine mountings were designed with a split-segment to facilitate rapid engine removal without disturbing the carburettors. A pair of fuel tanks, each containing up to , were housed within the centre-section of the fuselage.
The fuselage of the Blenheim employed a light-alloy monocoque structure using open-section stringers, and was constructed in three sections. The wing is also built in three sections, the centre-section of which is bolted and rivetted to the fuselage. The outer wing sections are tapered in chord and thickness. Extensive use of Alclad sheeting is made in elements such as the ribs, skin, flaps, and web reinforcement of the spars. The tail unit is of a cantilever monoplane style, using an all-metal tailplane and fin while the aerodynamically balanced rudder and elevators use a metal frame covered with fabric. The undercarriage was hydraulically-retracted, with an auxiliary hand-pump for emergency actuation; medium-pressure tyres were used, complete with pneumatically-actuated differentially-control brakes.Oughton 1971, p. 177.