Originally, the Bristol Blenheim was not created as a bomber or with the RAF in mind. During 1933 Bristol Chief Designer Frank Barnwell announced a proposal for a high-speed light passenger aircraft, the Bristol Type 135. The Type 135 as envisioned as a low-wing monoplane capable of carrying up to eight passengers within an all metal cantilever stressed skin fuselage, powered by two 500 hp (373 kW) nine cylinder Bristol Aquila I sleeve-valved air cooled radial engines. By 1934 work on the design had advanced to the fuselage mock-up stage and it was decided to display the mock up at the 1935 Salon Internationale de L'Aeronautique in Paris.

In 1934 Lord Rothermere, who was the owner of the Daily Mail newspaper, expressed a desire to obtain for his personal use, a fast and spacious private aeroplane, for this aviation-minded organisation had then appreciated the potential of what is today called the business or corporate aircraft. Lord Rothermere envisaged his requirements as a fast aircraft that would accommodate a crew of two and six passengers, and it just so happened that the Bristol Aeroplane Company had already drawn up the outline of a light transport in this category, the Type 135.

The new aircraft had been designed originally to be powered by two 500 hp (373 kW) Bristol Aquila I engines which were then under development. The Type 135 had an anticipated top speed of 180 mph (290 km/h) but lacked the range to meet Lord Rothermere's requirements. Frank Barnwell proposed changes that included reducing the fuselage cross section to reduce drag and replacing the 500 hp (373 kW) Bristol Aquila I engines with a couple of 640 hp (477 kW) Bristol Mercury VI radial engines driving fixed pitch four blade propellers. Design work started on the now designated Bristol Type 142 with Lord Rothermere as its principle source of funding. It would cost him £18,500 to complete the aircraft, a large sum even by todays standards. Bristol had learned of government plans to expand the RAF and with the anticipation of possible future contracts decided to fund a parallel design called the Bristol Type 143 as a private venture. The Type 143 featured a longer nose and longer undercarriage doors.

First flown at Filton on 12 April 1935, the Type 142 was to spark off much comment and excitement when during its initial trials it was found to be some 30 mph (48 km/h) faster than the prototype of Britain's most-recently procured new biplane fighter, the Gloster Gauntlet. Named Britain First, it was presented to the nation by Lord Rothermere after the Air Ministry had requested that they might retain it for a period of testing to evaiuate its potential as a light bomber. It had a number change from G-ABCZ to K-7557 and was then moved to Martlesham Heath for RAF trials. It proved so successful that in 1935 the Air Ministry issued Specification B.28/35 for a military version with similar performance. This, then was the sire of the Bristol Blenheim which was to prove an important interim weapon at the beginning of World War II.

The Bristol Mk IV dorsal turret on a Blenheim Mk IV aircraft.

Aware of Air Ministry interest in the Type 142, Bristol busied themselves with homework to evolve a military version (Type 142M) of this aircraft, and in the summer of 1935 the Air Ministry decided to accept the company's proposal, placing a first order for 150 aircraft to Specification B.28/35 in September. The new aircraft was very similar to the Type 142, but there had of course been some changes to make it suitable for the military role, primarily to accommodate a bomb aimer's station, a bomb bay and a dorsal gun turret. Little time was lost by either the Bristol company or the Air Ministry, for following the first flight of the prototype, on 25 June 1936, it was moved to Boscombe Down on 27 October 1936 for the start of official trials, with initial deliveries to RAF squadrons beginning in March 1937. In July 1937 the Air Ministry placed a follow-on order for 434 additional Blenheim Mk Is, as the type had by then been named.

Of all-metal construction, except for fabric-covered control surfaces, the Blenheim Mk I was a cantilever mid-wing monoplane, with the wing having Frise mass-balanced ailerons and split trailing-edge flaps. The fuselage nose extended only slightly forward of the engines, and both fuselage and tail unit were conventional light alloy structures. Landing gear was of the retractable tailwheel type. The tailwheel of the prototype had retracted, operated by cables linked to the main landing gear but, wisely, this feature was not carried forward into the production aircraft. The powerplant comprised two Bristol Mercury VIII engines developing 730 hp (545 kW) for take-off with a maximum power rating of 840 hp (626 kW) in level flight, mounted in nacelles on the wing leading-edge, and driving three-blade variable-pitch propellers. Accommodation was provided for a pilot, navigator/bomb-aimer, and air gunner/radio operator. A bomb bay in the wing centre-section could contain a maximum 1,000 Ibs (454 kg) of bombs, and standard armament comprised a 7.7 mm (0.303 in) machine-gun in the port wing, plus a single Vickers 'K' machine-gun in a dorsal turret.

Initial deliveries of production Blenheim Mk Is to the RAF squadrons began in March 1937. The first aircraft (K7036) to be delivered, however, crashed upon landing totally destroying the aircraft. The first RAF squadron to receive Blenheim Mk Is was No.114, then based at RAF Wyton, and it was this unit which first demonstrated the new type officially to the public at the RAF's final Hendon Display in the summer of 1937. The Blenheims were to arouse excited comment with their high speed and modern appearance, being launched on their career in an aura of emotion created by the belief that, in an unsettled Europe, the RAF was armed with the world's most formidable bomber aircraft. Production contracts soared, necessitating the establishment of new construction lines by A. V. Roe at Greengate, Middleton (Chadderton) and Rootes Securities at Speke (South Liverpool), both these factories being in Lancashire. Between them the three lines built a total of 1,355 Blenheim Mk Is which, at their peak, equipped no fewer than 26 RAF squadrons at home and overseas, the Blenheim's first overseas deployments being with No.30 Squadron in Iraq and No.11 Squadron in India, in January and July 1938 respectively.

However, by the outbreak of World War II few Blenheim Mk Is remained in service with home-based bomber squadrons, having been superseded in the bombing role by the Blenheim Mk IV, which incorporated the lessons learned from the experience which squadrons had gained in operating the Mk I. But their usefulness was by no means ended, many continuing to serve as conversion trainers and, initially, as crew trainers in OTUs. More valuable by far were some 200 which were converted to serve as night fighters, pioneering the newly conceived technique of AI (Airborne Interception) radar, carrying AI Mk III or Mk IV. The single forward-firing machine-gun was totally inadequate for this role, of course, and a special underfuselage pack to house four 7.7 mm (0.303 in) machine-guns was produced. So equipped, Blenheim Mk IFs scored the first AI success against an enemy aircraft on the night of 2-3 July 1940.

Export versions of the Blenheim Mk I were sold before the war to Finland, Turkey and Yugoslavia, and were also built under licence by these first two nations. In addition, a small number had been supplied to Romania as a diplomatic bribe in 1939, but this proved to be unsuccessful. The result, of course, was that Blenheim Mk Is fought for and against the Allies.

When, in August 1935, the Air Ministry had initiated Specification G.24/35 to find a successor to the Avro Anson for use in a coastal reconnaissance/light bomber role. Bristol had proposed its Type 149. Very similar to the Blenheim Mk I, this was based on the use of Bristol Aquila engines to confer long range with the existing fuel capacity, but proved unacceptable to the Air Ministry. Subsequently renewed interest was shown in the Type 149 for use in a general reconnaissance role, and a prototype was built, by conversion of an early Blenheim Mk I, this retaining the Mercury VIII engines and being provided with increased fuel capacity. The fuselage nose was lengthened to provide additional accommodation for the navigator/observer and his equipment, and this was to be finalised as that which graced the Blenheim Mk IV.

The Air Ministry then had misgivings about the Type 149, fearing that its introduction and manufacture would interfere with the production or urgently needed Blenheims. Instead, the Type 149 was adopted by the Royal Canadian Air Force and with the start of the British Commonwealth Air Training Plan, it was decided to produce a version in this country. Fairchild Aircraft of Longueil outside Montreal was selected to produce them under the Canadian name Bolingbroke. Quickly nicknamed the Boly, the type saw service throughout Canada. The Bristol prototype being shipped to Canada to help in the establishment of a production line. The first Bolingbroke Is had Mercury VIII engines, but after 18 of these had been built production changed to the definitive Canadian version, the Bolingbroke IV with Mercury XV engines, and equipment from both Canadian and US manufacturers. Later variants included a small number of Bolingbroke IV-Ws with American built Pratt and Whitney Twin Wasp Junior (SB4-G) 14-cylinder engines rated at 825 hp (615 kW) for take-off, and a number of Bolingbroke IV-T multi-purpose trainers.

Having blown hot and then cold over the Type 149, there was a sudden renewal of interest, primarily as an interim measure until the Type 152 torpedo-bomber, derived from the Blenheim, should become available. The decision was taken, therefore, to introduce the longer nose and stepped windscreen of the Bolingbroke, and to make provision for longer range by the introduction of increased wing fuel capacity. The Bristol designation Type 149 was retained for this changed configuration, the new RAF designation being Blenheim Mk IV. This change took place quietly on the production lines towards the end of 1938, although the first 68 Blenheim Mk IVs were built without the 'Iong-range wing'. The powerplant comprised two more powerful Mercury XV engines, and these allowed gross weight to be increased eventually by 16 per cent.

No. 90 Squadron was the initial unit to be equipped with Blenheim Mk IV s in March 1938, the first of more than 70 squadrons to operate these aircraft, and consisting of units from Army Co-operation, Bomber, Coastal, Far East Bomber, Fighter and Middle East Commands, both at home and overseas. Inevitably, such extensive use brought changes in armament and equipment, but especially the former, for the armament of the first Blenheim Mk IVs was unchanged from the initial two-gun armament of the Mk I. As finalised the number became five, the single forward-firing gun in the wing being retained, a new dorsal turret carrying two guns being adopted, and a completely new remotely-controlled Frazer-Nash mounting being added beneath the nose to hold two aft-firing machine-guns. Protective armour was also increased, but while it was not possible to enlarge the capacity of the bomb bay, provision was made for an additional 320 Ibs (145 kg) of bombs to be carried externally, under the inner wings, for short-range missions.

With so many squadrons operating the type it was inevitable that Blenheims should notch up many wartime 'firsts' for the RAF. These included the first reconnaissance mission over German territory, made on 3 September 1939. It was flown by Flying Officer A Macpherson in a Blenheim (N6215) Mk IV of No. 139 Squadron while on an armed reconnaissance over German warships at Heligoland Bight (Schillig Roads) near Wilhelmshaven. On 4 September 1939, ten aircraft from Nos. 107 & 110 Squadrons, led by Flight Lieutenant K.C. Doran of No. 110 Squadron made an attack on the same German ships. From the beginning of the war, until replaced in home squadrons of Bomber Command by Douglas Bostons and de Havilland Mosquitoes in 1942, Blenheim Mk IVs were used extensively in the European theatre. Although vulnerable to fighter attack, they were frequently used for unescorted daylight operations and undoubtedly the skill of their crews and the aircraft's ability to absorb a great deal of punishment were the primary reasons for their survival, for high speed and heavy firepower was certainly not their forte. In the overseas squadrons Blenheims continued to serve long after their usefulness had ended in Europe, and except in Singapore, where they were no match for the Japanese fighters, they proved a valuable weapon. A total of 3,298 Mk IV had been built in England when production ended, and in addition to serving with the RAF had been used by the French Free and South African air forces, and supplied in small numbers to Finland, Greece and Turkey.

Last of the direct developments of the Blenheim design was Bristo1's Type 160, known briefly as the Bisley, which was to enter service in the summer of 1942 as the Blenheim Mk V. Envisaged originally as a low-altitude close-support bomber, it was in fact to be built for deployment as a high-altitude bomber, powered by Mercury XV or XXV engines. Except for a changed nose, some alterations in detail and updated equipment, these aircraft were basically the same as their predecessors. Some 942 were built, all produced by Rootes at their Speke (South Liverpool) and Blythe Bridge (Stoke-on-Trent) factories, and the first unit to receive Blenheim Mk Vs was No.18 Squadron. The type was to equip six squadrons in the Middle East and four in the Far East, where they were used without distinction. This resulted from an increase in gross weight of over 17 per cent which, without the introduction of more powerful engines, had brought about a serious fall of performance. It only when the Blenheim Mk Vs were deployed in Italian campaign, contending with the advanced fighters in service with the Luftwaffe, that losses rose to quite unacceptable proportions, and the Blenheim Mk Vs withdrawn from service.

Canadian Bolingbrokes

Operational use of the Bolingbroke was limited to the Royal Canadian Air Force in Canada and the Aleutian Islands. No 8 (Bomber Reconnaissance) Squadron was the first RCAF unit to convert to the Bolingbroke, followed by one other squadron. Bolingbrokes were used primarily to fly anti-submarine coastal patrols over both the Atlantic and Pacific.

Two RCAF squadrons were assigned to the combined American-Canadian defence campaign to protect the Aleutian Islands and west coast of Alaska from Japanese attack. No. 115 Squadron arrived in the Aletuians in April of 1942 and was assigned anti-submarine patrol and maritime reconnaissance missions. In June of 1942 No. 8 Squadron deployed to the Aleutians with twelve Bolingbroke Mk IVs, making a 1,000 mile flight from RCAF Sea Island to Yakutat Island arriving on 3 June. When the squadron arrived it was ordered to paint out the Red centers to the upper wing roundels to prevent confusion with the Japanese 'meatball' insignia. Later additional recognition markings in the form of a fourteen inch Blue band was added to the rear of the fuselage. The harsh weather in the Aleutians proved a worse enemy than the Japanese and a number of Bolingbrokes were lost when thick Alaskan fogs obscured mountain tops. Normal bomb loads consisted of three 300 pound depth charges and two aircraft were maintained in an alert status at all times. The squadron is credited with sharing one submarine kill with the US Navy. A Bolingbroke Mk IV piloted by Flight Sergeant P.M.G. Thomas attacked and damaged a Japanese submarine enabling US Navy surface units to later sink it.

The majority of Bolingbrokes produced never saw combat, instead they performed as crew and operational trainers under the Commonwealth Air Training Plan, training crews for overseas units. Still others were converted to unarmed target tugs with high visibility paint schemes for training air gunners and army anti-aircraft gunners.

Specifications (Bristol Type 149 Blenheim Mk IV)

Type: Three Seat Light Bomber, Fighter & Night Fighter, Maritime Reconnaissance (Anti-Shipping/Submarine), Bombing and Gunnery Trainers & Target Tug.

Accommodation/Crew: Pilot, Navigator/Bomb-Aimer and Wireless Operator/Air Gunner. See the Blenheim Mk I for more cockpit information.

Design: Chief Designer Frank Barnwell of the Bristol Aeroplane Company Limited.

Manufacturer: The Bristol Aeroplane Company Limited based at Filton (Bristol), Bristol County, England (Mark I, IV & V Prototypes), Alexander V. Roe (Avro) Aircraft Company Limited based in Greengate, Middleton (Chadderton), Lancashire County, England (Mark I & IV), Rootes Securities Limited at Blythe Bridge (Stoke on Trent), Straffordshire County, England (Mark IV & V), Rootes Securities Limited at Speke (South Liverpool), Lancashire County, England (Mark I & V), Fairchild Aircraft Limited in Longueil, Quebec, Canada (Bolingbroke). Also built under licence by Valtion Lentokonetehdas (State Aircraft Factory) at Tampere, Finland (Mark I & IV) and Ikarus AD in Belgrade (Zemun), Yugoslavia (Mark I).

Powerplant: (100 Octane Fuel) Two Bristol Mercury XV 9-cylinder poppet-valve air-cooled radial engines developing 905 hp (675 kW) at take-off, a maximum output of 995 hp (742 kW) for level flight (5 minute usage) and a maximum ecomical cruising power output of 590 hp (440 kW) at 16,000 ft (4877 m) at 2400 rpm. (87 Octane Fuel) Two Bristol Mercury XV 9-cylinder poppet-valve air-cooled radial engines developing 725 hp (541 kW) at take-off, a maximum output of 840 hp (627 kW) for level flight (5 minute usage) and a maximum ecomical cruising power output of 590 hp (440 kW) at 16,000 ft (4877 m) at 2400 rpm.

Performance: Maximum speed 266 mph (428 km/h) at 11,800 ft (3595 m); cruising speed of 198 mph (319 km/h); service ceiling (clean) 27,260 ft (8310 m) or 22,000 ft (6706 m) fully loaded; initial climb rate 1,480 ft/min (7.5 m/sec).

Fuel Capacity: Two inboard 140 Imperial gallon (636 litres) main fuel tanks and two outboard 94 Imperial gallon (427 litres) auxiliary or long range fuel tanks giving a total capacity of 468 Imperial gallons (2125 litres). Starting in early 1940 the main fuel tanks were self-sealing, but due to an initial shortage, the outboard auxiliary fuel tanks remained non self-sealing for some time.

Oil Capacity: One 11.5 Imperial gallon (52.2 litre) main oil tank and a 2.5 Imperial gallon (11.3 litre) auxiliary oil tank per engine giving a total oil capacity of 28 Imperial gallons (127.2 litres).

Range: 1,460 miles (2350 km) on internal fuel with a 1,000 lbs (454 kg) bombload. 1,950 miles (3140 km) on internal fuel without bombs.

Weights & Loadings: Empty 9,790 lbs (4441 kg), with a normal take-off weight of 13,500 lbs (6122 kg) and a maximum take-off weight of 14,400 lbs (6532 kg) fully loaded with bombs.

Dimensions: Span 56 ft 4 in (17.17 m); length 42 ft 7 in (12.98 m); height 9 ft 10 in (3.00 m); wing area 469 sq ft (43.57 sq m).

Defensive Armament: A total of three to five 7.7 mm (0.303 in) machine-guns was standard. Some Mk IV aircraft underwent various field modifications with further increased the aircrafts defensive armament. The Browning machine-guns were belt feed while the Vickers machine-guns used 50 round circular ammunition pans. The Frazer-Nash FN.54 and FN.54A turrets were jettisonable in the event of an emergency allowing the crew to use the lower fuselage emergency escape hatch.

• 1 × 7.7 mm (0.303 in) Browning fixed forward-firing machine-gun in the port wing.
   

• 1 x 7.7 mm (0.303 in) Lewis or Vickers "K" trainable machine-gun in a semi-retractable hydraulically operated Bristol B.Mk III dorsal turret, or
   

• 2 x 7.7 mm (0.303 in) Vickers "K" trainable machine-guns in a power-operated Bristol B.Mk IIIA dorsal turret, or
   

• 2 x 7.7 mm (0.303 in) Browning trainable machine-guns in a power-operated Bristol B.Mk IV dorsal turret.
   

• 1 × 7.7 mm (0.303 in) Browning trainable rearward-firing machine-gun in a remotely controlled Frazer-Nash FN.54 chin turret, or
   

• 2 × 7.7 mm (0.303 in) Browning trainable rearward-firing machine-guns in a remotely controlled Frazer-Nash FN.54A chin turret. The turret could rotate 20 degrees to either side with a depression of 17 degrees.
   

• 1 × 7.7 mm (0.303 in) Vickers "K" forward-firing machine-gun in a gimbal nose gun mount (optional field modification).
   

• 1 × 7.7 mm (0.303 in) Vickers "K" machine-gun in a rear firing engine nacelle mount (optional field modification).
   

• 1 × 7.7 mm (0.303 in) Vickers "K" machine-gun in a rear firing under tail mount (optional field modification).

Offensive Armament: 1,000 lbs (454 kg) of bombs internally and up to 320 lbs (145 kg) of bombs externally on two underwing racks located between the fuselage and engine nacelles. On the single Mk II aircraft produced, 500 lbs (227 kg) of bombs could be carried externally but at great expense to performance.

• 4 × 250 lbs (114 kg) bombs, or
   

• 2 × 500 lbs (227 kg) bombs, or
   

• 3 × 300 lbs (114 kg) depth charges carried internally.
   

• 4 × 80 lbs (36.2 kg) bombs on underwing racks, or
   

• 2 × 160 lbs (72.5 kg) bombs on underwing racks

Variants: Bristol Type 142, Bristol Type 142M, Bristol Type 143 (Aquila engined), (Type 142M) Blenheim Mk I Prototypes, (Type 142M) Blenheim Mk I, Blenheim Mk IF, Blenheim PR.Mk I, Blenheim Mk II, Blenheim Mk III, Bristol Type 149, (Type 149) Blenheim Mk IV Prototypes, (Type 149) Blenheim Mk IV, (Type 149) Blenheim Mk IVF, (Type 149) Blenheim Mk IVL, (Type 160) Bisley Mk I, (Type 160) Blenheim Mk V, Bolingbroke Mk I, Bolingbroke Mk II, Bolingbroke Mk III, Bolingbroke Mk IV, Bolingbroke Mk IV-C, Bolingbroke Mk IV-W, Bolingbroke Mk IV-T, Bolingbroke Mk IV-TT

Equipment/Avionics: Standard communications and navigation equipment.

History: First flight (Type 142 "Britain First") 12 April 1935; first flight (Type 142M) 25 June 1936; initial delivery (No. 114 Squadron RAF) March 1937; end production (VD) June 1943; withdrawn from service (Finland) 1956.

Operators: United Kingdom (RAF), Canada (RCAF), Finland, Turkey, Yugoslavia, Romania, Greece (Royal Hellenic Air Force), Free French Air Force (Forces Francaises Libre), Portugal (Arma de Aeronautica), South Africa (SAAF) and Croatia. The German Luftwaffe and the Italian Regia Aeronautica both operated captured aircraft.

Number Built: 4,422

Number Still Airworthy: One