In 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. 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 in Europe. Bristol had been working on a suitable design as the 'Type 135' since July 1933, and further adapted it to produce the Type 142 to meet Rothermere's requirements.
Named 'Britain First', this first flew at Filton on 12 April 1935, and proved to be faster than any fighter in service with the Royal Air Force at the time. The Air Ministry was obviously interested in such an aircraft and quickly sent out Specification B.28/35 for prototypes of a bomber version; the 'Type 142M' (M for military). The main change was to move the wing from a low-wing to a mid-wing position, allowing room under the main spar for a bomb bay. The aircraft was all-metal with two Bristol Mercury VIII air-cooled radial engines, each of 860 hp (640 kW). It carried a crew of three – pilot, navigator/bombardier and telegraphist/air gunner. Armament comprised a single forward-firing .303 in (7.7 mm) Browning machine gun outboard of the port engine and a .303 in (7.7 mm) Lewis Gun in a semi-retracting Bristol Type B Mk I dorsal turret firing to the rear. From 1939 onwards, the Lewis gun was replaced by the more modern .303 in (7.7 mm) Vickers VGO machine gun of the same calibre. A 1,000 lb (450 kg) bomb load could be carried in the internal bay.
To achieve its relatively high speed, the Blenheim had a very small fuselage cross-section, with its upper front glazing all at one angle in the form of a "stepless cockpit", that used no separate windscreen panels for the pilot, a notable feature of a substantial majority of German bomber designs, first conceived during the war years. The pilot's quarters on the left side of the nose were so cramped that the control yoke obscured all flight instruments while engine instruments eliminated the forward view on landings. Most secondary instruments were arranged along the left side of the cockpit, with essential items like propeller pitch control actually placed behind the pilot where they had to be operated by feel alone. Like most contemporary British aircraft, the bomb bay doors were kept closed with bungee cords and opened under the weight of the released bombs. Because there was no way to predict how long it would take for the bombs to force the doors open, bombing accuracy was consequently poor.
The aircraft was ordered directly from the drawing board with the first production model serving as the only prototype. The service name then became Blenheim Mk I after the famous battle during the War of the Spanish Succession. Subsequent deliveries started on 10 March 1937, with 114 Squadron being the first squadron to receive the Blenheim. The aircraft was built under license by countries including Finland and Yugoslavia, which built 60 aircraft. Other countries bought it, including Romania, Greece and Turkey. Total production of the Blenheim Mk I in England was 1,351 aircraft.
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), but only one was completed. 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. 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. Their performance was marginal as a fighter, but they served as an interim type, pending availability of the Beaufighter. 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.
- Blenheim Mk I : Three-seat twin-engined light bomber, powered by two 840 hp (630 kW) Bristol Mercury VIII radial piston engines, armed with a 0.303 in (7.7 mm) machine gun in the port wing, plus a 0.303 in (7.7 mm) Vickers K gun in the dorsal turret, maximum bombload 1,000 lb (450 kg). 1,552 built. Company designation Type 142M.
- Blenheim Mk IF : Night fighter version, equipped with an AI Mk III or Mk IV airborne interceptor radar, armed with four 0.303 in (7.7 mm) machine guns in a special gun pack under the fuselage. About 200 Blenheim Mk Is were converted into Mk IF night fighters.
- Blenheim Mk II : Long-range reconnaissance version with extra fuel tankage. Only one Blenheim Mk II was built.
- Blenheim Mk III
- Blenheim Mk IV : Improved version, fitted with protective armour, powered by two 905 hp (675 kW) Bristol Mercury XV radial piston engines, armed with a 0.303 in (7.7 mm) machine gun in the port wing, plus two 0.303 in (7.7 mm) machine-guns in a powered operated dorsal turret, and two remotely controlled rearward-firing 0.303 in (7.7 mm) machine gun mounted beneath the nose, maximum bombload 1,000 lb (450 kg) internally and 320 lb (150 kg) externally. 3,307 built.
- Blenheim Mk IVF : Long-range fighter version, armed with four 0.303 in (7.7 mm) machine guns in special gun pack under the fuselage. About 60 Blenheim Mk IVs were converted into Mk IVF fighters.
- Blenheim Mk V : High-altitude bomber version, powered by two Bristol Mercury XV or XXV radial piston engines.