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History

The Bristol Blenheim is a British three-seat light fast bomber of the Second World War. Originating as a civilian project, this twin-engine aircraft was the first cantilever-winged metal monoplane to enter service with the RAF. Faster than most of the fighters in service in 1936, it proved disappointing in the first combat trials in 1939, but was an important part of the British military system at the beginning of World War II.
In 1934, the Bristol Aeroplane Company presented at the Paris International Air Show a full-scale model of the fuselage of a twin-engine fast transport aircraft on which Frank Barnwell, head of the Bristol design office, had been working since July 1933. A low-wing cantilever monoplane of all-metal construction with a working skin, this aircraft was to carry six to eight passengers in a closed cabin at 290 km/h. As there were no engines compatible with this airframe, Bristol also announced the development of a nine-cylinder star engine of 350 hp called Aquila.
The Harmsworth brothers were owners of major English newspapers, such as The Daily Mail, The Daily Mirror and The Times. The youngest, Lord Rothermere, had always followed aviation developments closely, even serving as Secretary of State for the Royal Air Force in the Lloyd George government. When Lord Beaverbrook, another British newspaper magnate, bought the DC-1, Lord Rothermere felt compelled to inform the Bristol Aeroplane Company that he could buy a Bristol 135 if he could fly non-stop to major European cities. But the Bristol 135 did not have enough range. Frank Barnwell therefore modified his initial project.

The new project, dated April 27, 1934, was for a monoplane equipped with Bristol Mercury VI engines of 640 hp and capable of reaching 400 km/h. The Bristol 135’s wing was kept, but the fuselage was thinner thanks to a monocoque structure allowing to lodge six passengers behind the two pilots, who had an excellent field of vision because of the particular shape of the front fuselage. The main landing gear lifted backwards into the engine nacelles.

The Bristol 142 prototype made its first flight on April 12, 1935 at Filton, flown by Captain Cyril Uwins, the manufacturer’s chief pilot, with test registration R-12 and four-bladed wooden propellers. After being fitted with Hamilton Standard three-bladed variable pitch propellers, the prototype, named Britain First by its owner, underwent its certification tests at Martlesham Heath in June 1935. It as to rewach a speed of 459 km/h at full load, 48 km/h better than the last fighter ordered in series by the RAF, the Gloster Gladiator. Surprised by the result, the Air Ministry asked Lord Rothermere to study the aircraft. He offered his twin-engine aircraft to the Air Council. The unique Bristol 142 never carried the civil registration G-ADCZ that had been assigned to it.Officially taken over by the RAF in July 1935, the Bristol 142 received the British roundels and the serial number K7557 and was used for transport missions by the RAE at Farnborough until 1942.

In parallel to the Type 142, the Bristol design office developed a more basic and less expensive version to replace the Type 135. It was in fact a Bristol 142 with the Bristol Aquila III engines developed for the Type 135. The single Bristol 143 prototype made its first flight on January 20, 1936 with the temporary registration R-14 and, like the Bristol 142, was handed over to the RAF without ever having carried the civil registration G-ADEK that had been reserved for it. This aircraft was never used by the RAF, but was used as a testbed by the Bristol Aeroplane Company at Filton.

The Bristol 143 was the first to be developed for military use. As early as December 1934, this twin-engine aircraft was proposed to Coastal Command as a reconnaissance and coastal patrol aircraft. In January 1935, Finland was offered the Bristol 143F, a fighter-bomber equipped with Mercury IX engines, a Madsen 20 mm front gun and a dorsal turret. The Finnish government was negotiating the purchase of nine aircraft when the Bristol 142M project emerged.

Contrary to what has often been written, the transformation of the Bristol 142 from a twin-engine fast transport to a medium bomber was a significant engineering task: proposed on 9 July 1935 to the Ministry of Air, the project was for a two-seater bomber equipped with Aquila or Mercury engines, which was to carry 450 kg of bombs over 1,600 km. Compared to the Bristol 142, the wing was raised by 41 cm to allow the installation of a bomb bay under the longitudinal spars that could hold four 110 kg bombs or two 225 kg bombs. The horizontal stabilizer was enlarged and raised, the introduction of trim-tab on each elevator replaced the incidence adjustment device, and the vertical stabilizer was lengthened by 20 cm. The rear wheel became retractable, cabled to the main gear to avoid any hydraulic equipment at the rear of the fuselage. Finally, the front fuselage was modified (but not lengthened) to accommodate a bombardier station and a 7.7 mm Browning machine gun, and a semi-retractable dorsal turret was envisaged. Including structural reinforcements and military equipment, the empty weight was increased by 10% and the total loaded weight by 24%.
The tests carried out with the Britain First having given a precise idea of what the future aircraft would be, it did not seem necessary to the Air Ministry to go through a prototype phase and in August the B.28/35 specification was established to cover the definition of the aircraft, an order for 150 aircraft notified on plans and the new bomber named Blenheim. The RAF had high expectations of the twin-engined aircraft and intended to move very quickly, and the first two production aircraft were to be used for developmental testing.
The first production aircraft K7033 made its maiden flight on 25 June 1936 and by December a further order for 434 twin-engined aircraft was placed with the Bristol Aeroplane Company with instructions to speed up production. A year later 25 Blenheims were rolling out of the Filton factory every day, while two more assembly lines were opened, at Rootes Securities Ltd in Speke (422 aircraft ordered) and A.V.Roe & Co Ltd in Chadderton (250 aircraft ordered). In 1937 a final order for 134 aircraft was placed at Bristol.

Variants

Mk I
The first production aircraft K7033 began testing with 840hp Bristol Mercury VI-S.2 engines and during its tests at Martlesham Heath reached a speed of 452km/h at 3,657m despite weighing 900kg more than the Bristol 142 when loaded. This aircraft was quickly modified and immediately introduced into production: the engines were replaced by 840hp Mercury VIIIs, the right wing landing light was removed and the cockpit side window was modified. A little later, the propeller spinner was removed and the tailwheel became fixed. The Mk I was equipped for three men (a pilot, a navigator-bombardier and a radio gunner) and was armed with a 7.7 mm Browning machine gun in the left wing and a Vickers K of the same calibre in a hydraulically operated Bristol dorsal turret. The first aircraft to enter operational service K7035 was delivered to No 114 Sqn stationed at Wyton in early 1937, which was declared operational on 10 March 1937, just four years after Barnwell had drawn up the first Type 135 designs on paper. The Blenheim’s participation in the annual Hendon Airshow, also in 1937, caused a sensation, and the delivery of aircraft to squadrons based in Iraq or India was the subject of some spectacular flights, such as a group flight from Dhibban to Aboukir, covering 1,306 km in 3.25 hours. When the Munich crisis broke out, there were 15 RAF squadrons operating in the UK with Blenheim Mk I’s, which were replaced in the front line by Mk IV’s in September 1939. Some 200 of these were converted to night fighters, the remainder going to reinforce RAF equipment in the Middle East or being exported.
Also, a Mk I L1348 was modified as a photographic reconnaissance aircraft without being given a specific designation. The wing was reduced by 91cm, the turret removed, the glass panels at the base of the forward fuselage obscured, and various photographic equipment installed in the fuselage. With ‘souped-up’ engines, this aircraft reached 476 km/h at 2,440 m.

Mk IF
Some 200 Mk I bombers were modified as twin-engine escort fighters (hence the F for Fighter) with four fixed 7.7mm Browning machine guns housed in a belly fairing with 500 rounds per gun, the first aircraft being commissioned by No 25 Squadron in December 1938. By September 1939 six fighter squadrons were equipped or being equipped. The aircraft proved to be a poor daytime fighter, an easy target for enemy single-seaters, but an excellent night fighter, equipped successively with the AI Mk II, III and IV interceptor radars. The first night sorties took place at the end of December 1939, and the first successful interception was made on the night of 22/23 July 1940 with an AI Mk IV radar.

Mk II
A single Mk I L1222 was fitted with larger wing tanks, the structure being reinforced to allow a load weight of 6,340kg and the possibility of carrying bombs under wing.

Mk III
It seems that this designation was given to a proposed Blenheim IV which would have retained the Mk I wing (thus similar to the Blenheim IVL). Like the previous one, this model was not given a Type number and some authors continue to claim that the Bristol 149 Bolingbroke prototype K7072 was originally named Blenheim III, an error deliberately peddled during the war for propaganda purposes.

Mk IV
From 1935 Frank Barnwell worked on various evolutions of the Type 142M. To meet the M.15/35 specification the Type 150 was a Bristol Perseus VI powered torpedo bomber carrying a torpedo in the belly hold. The G.24/35 specification was covered, as we have seen, by the Type 149 Bolingbroke, but in April 1935 development began on a new version of the Blenheim to meet both the G.24/35 (twin-engined multipurpose) and M.15/35 (torpedo bomber) specifications. This project led to the Bristol 152 Beaufort, but Coastal Command could not wait and the Beaufort’s development was in danger of being disrupted by the accidental death of Frank Barnwell in a light aircraft he had built himself and which was making its second flight.

The Air Ministry took a renewed interest in the Bolingbroke and decided to revive production of the aircraft, which retained the Type 149 but was christened Mk IV. The Blenheim IV was a Blenheim Mk I with the front fuselage of the Bolingbroke and the wing of the Blenheim II, with new wing tanks to increase range. There was no prototype and the Mk I was upgraded to the Mk IV at the end of 1938 by simply modifying 68 Mk I airframes on the production line.

Blenheim IVL: 68 Mk I airframes modified on the production line, thus without the wing tanks of the standard model.
Blenheim IVF: Night fighter corresponding to the Mk IF, the armament being identical. Approximately 60 examples were modified in this way, used mainly by thirteen RAF fighter squadrons.
Blenheim IV: Standard model with Mercury XV engine of 920 hp at take-off. As with the Mk I the payload capacity was 454 kg, but an additional 145 kg could be carried under the wing. The defensive armament was in principle identical to that of the Mk I, but from the first fights it appeared necessary to improve the defensive armament of this bomber. Standard modifications introduced during production included the installation of a Vickers K machine gun pivoted in the nose cone and a Browning machine gun installed in a glass gondola under the front fuselage, at the location of the crew escape hatch, and firing towards the rear. The last production aircraft received a Bristol B.I Mk.IIIA dorsal turret with two Vickers K or B.I Mk IV with two Browning guns. A remote-controlled Frazer-Nash twin with two rear-firing Browning guns was finally fitted under the nose of some aircraft. More or less effective modifications were also made in squadrons, such as fixed rear-firing machine guns installed either in the engine nacelles or in the rear fuselage tip.

Mk V
In January 1940, Bristol proposed to the Air Ministry a version of the Type 149 Blenheim IV specially adapted for tactical support, with the nose section modified to accommodate four 7.7mm Browning machine guns (1,000 rounds each). This project took into account the experience gained during the first months of the conflict in Europe, as the aircraft could also be used as a fighter or as a trainer. As the Air Staff was already considering this type of aircraft, the B.6/40 specification was written around this definition to allow the order of two Bristol 160 Bisley Mk I prototypes, which were built by Rootes Securities, Bristol’s main subcontractor on the Blenheim programme.
In addition to the modified front fuselage, the cockpit was armoured, the windscreen redesigned, a Bristol B.X dorsal turret equipped with two Browning machine guns was installed and the engines were changed to Mercury XVIs for better low-level performance. While the prototypes were being built, the programme was modified to allow an alternative use as a very high altitude bomber. It was therefore decided to make an interchangeable nose cone: for high-altitude missions, it was glazed on the left side to accommodate a bomber-navigator and received a lower fairing to allow it to sit, but also to accommodate a Frazer-Nash barbette with two rear-firing Browning machine guns. The engines became Mercury XV or XXV of 830 hp, insufficient for an aircraft whose total weight reached 7,700 kg.
The designation Blenheim V replaced the Bisley designation before the first Bristol 160 AD657 took to the air at Filton on 24 February 1941 with the nose of the ground attack version, the second prototype AD661 being equipped as a high altitude bomber. The low-altitude version was soon abandoned and 940 production aircraft with Mercury 25 or 30 engines were built by Rootes Securities Ltd at Blythe Bridge until June 1943. The first examples were delivered to No 18 Sqdn in the summer of 1942, with the aircraft entering operations in November in North Africa. The Blenheim V had a very short career, ending in late 1943 in the Far East. A number of them were modified in dual control without a dorsal turret for Fighter Command’s OTUs, some of them in service in the Mediterranean were transferred in 1942 to Turkey and some to Portugal, which already had a number of Mk IVs interned after forced landings on its territory.

Bolingbroke
To meet the G.24/35 twin-engine multi-role specification (coastal reconnaissance and bombing), the Type 149 was a slightly modified Mk I with Aquila AE-3M engines allowing a greater range. After extending the front of the fuselage by 91 cm to install the radionavigator in front of the pilot, 150 were ordered in November 1936 under the name Bolingbroke. In 1937, a Blenheim I K7072 was taken off the line and converted into the Bristol 149 Bolingbroke prototype (not Mk III).
The first step was to move the bomber’s windscreen and cockpit forward by 90cm without changing the pilot’s position. But before the start of the flight tests it was realised that the pilot would have a serious lack of visibility. The windscreen was therefore moved back to its original position and the glass roof of the front seat lowered below the pilot’s line of vision. Thus modified, it took to the air on 24 September 1937, but the forward visibility on landing was still poor. The upper left section was therefore lowered, giving a very characteristic look to the front of the aircraft.
The Royal Canadian Air Force wished to adopt the aircraft and negotiations began for overseas production, plans to build the Bolingbroke at Filton for RAF use being abandoned in favour of continuing production of the Mk I for overseas units. The Bolingbroke prototype K7072 was therefore shipped to Canada to facilitate the start of production at Fairchild Aircraft Ltd in Longueuil.
Bolingbroke I: 18 aircraft produced to plans supplied by Bristol Aeroplane Company, substantially identical to K7072 with Mercury VIII engines.
Bolingbroke II: A Mk I rebuilt to Mk IV standard after an accident.
Bolingbroke III: It was envisaged that the Bolingbroke could be fitted with skis or floats instead of the original landing gear to suit Canadian conditions. But in fact the idea was abandoned after the modification of a Bolingbroke I “717” with two EDO floats.
Bolingbroke IV: Production version redesigned to North American standards for use by the Canadian or American air forces. 125 were built with Mercury XV engines.
Bolingbroke IVC: A Mk IV completed with 990hp Wright R-1820-G3B Cyclone nine-cylinder star engines.
Bolingbroke IVW: 15 aircraft delivered with 14-cylinder Pratt & Whitney Twin Wasp R1830 1200 hp star engines.
Bolingbroke IVT: 517 aircraft for training bombers, gunners, twin-engine pilots, and even target towing.