Flying for the first time on 12 April 1935 the Bristol Type 142 was a twin-engine high-speed transport designed at the request of the press baron, Lord Rothermere. So startling was the performance that the Type 142, and later the Type 142M, were adopted for development as a light bomber under thej Air Ministry Specification B. 28/35. Thex result was the Bristol Blenheim Mk I, which was ordered straight from the drawing board. The first examples went to No, 114 Squadron at Wyton in March 1937. At the time of the Munich Crisis in September 1938 the Blenheim Mk I equipped 16 squadrons in Nos 1, 2 and 5 (Bomber) Groups of Bomber Command. As early as January 1938 the Blenheim Mk I entered service with No. 30 Squadron at Habbaniya, Iraq, while other Blenheim Mk Is were posted to AHQ India early in the following year. The Blenheim Mk I was powered by two 840-hp (626-kW) Bristol Mercury VIII radial engines. The light armament consisted of one 7,7-mm (0,303-in) machine-gun in the wing, and one manually-operated 707-mm (0.3-in) Vickers K gun in a dorsal turret; 454 kg (1,000 lb) of bombs could be carried. A total of 1,365 Blenheim Mk I bombers were produced by Bristol, Avrò and Rootes: 45 were made under licence by the Finnish VLT company, and the Ikarus firm of Yugoslavia made 16. With its characteristic short-nosed glazed canopy the Blenheim Mk I saw service with the RAF in Greece, Malaya and North Africa.
Engines of increased power and a longer, scalloped, nose characterized the main production variant, the Blenheim Mk IV, of which 3,286 were produced, Powered by two 920-hp (686-kW) Bristol Mercury XV radiais, the Blenheim Mk IV equipped seven squadrons in No. 2 (Bomber) Group at the outbreak of war in September 1939: armament was increased by the installation of two 7.7-mm (0.303-in) guns in a Bristol B.I. Mk IV dorsal turret, while a rearward-firing twin-gun turret could be installed under the nose section, sighted by a periscope. The Blenheim Mk IV scored a number of 'firsts' in World War II. On 3 September Blenheim Mk IV (N6215) of No, 139 Squadron, under Flying Officer A, McPherson, became the first RAF aircraft to enter German airspace and photograph the fleet units off Wilhelmshaven. On the following day Blenheim Mk IVs of Nos 107 and 110 Squadrons made the first offensive attack by Bomber Command. The RAF's first U-boat kill was made on 11 March 1940, by a Blenheim Mk IV of No. 82 Squadron flown by Squadron Leader M.V. Delap. Blenheim Mk IVs saw extensive service over France, off Norway, over Germany, Greece, Crete, North Africa, India, Malaya and Sumatra until August 1942 when they were phased out, Finland and Greece operated Blenheim Mk IVs, as did Canada where it was known as the Bolingbroke. The Blenheim Mk V (945 built) appeared in late 1942, powered by two 950-hp (708-kW) Mercury 25 or 30 engines, and saw service in North Africa and Tunisia, and in the Far East. Underpowered and poorly armed, the Blenheim lost more crews than any other RAF type.
Specification Bristol Blenheim B.Mk IV
Type: three-seat light bomber
Powerplant: two 920-hp (686-kW) Bristol Mercury XV radial piston engines
Performance: maximum speed 428 km/h (266 mph) at 3595 m (11,800 ft); cruising speed 318 km/h (198 mph); service ceiling 8310 m (27,260 ft); maximum range 2340 km (1,460 miles)
Weights: empty 4445 kg (9,790 lb); maximum take-off 6537 kg (14,400 lb)
Dimensions: span 17.17 m (56 ft 4 in); length 12.98 m (42 ft 7 in); height 2.99 m (9 ft 10 in); wing area 43.57 m2 (469 sq ft)
Armament: up to five 7.7-mm (0,303-m) machine-guns (one fixed in wing, two in dorsal turret, and two optional rearfiring), plus a normal bombload of 454 kg (1,000 lb)