The Type 135 civil twin design was on Bristol drawing boards by July 1933.
In 1934 Lord Rothermere, owner of the Daily Mail, issued a challenge to 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 high-speed designs that were breaking records, 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.
Blenheim cockpit. Note the asymmetry of the instrument console, which indicates the "scooped out" area of the nose in front of the pilot. The ring-and-post gunsight for the forward firing guns is also visible.When it first flew as Britain First at Filton on 12 April 1935, it 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 a aircraft, and quickly sent out Specification B.28/35 for prototypes of a bomber version of the Bristol called the Type 142M (M for "military"). The main changes were to move the wing higher on the fuselage from its former low position, to allow room under the spar for a bomb bay. The aircraft was all-metal with twin Bristol Mercury VIII radial engines of 860 hp (640 kW) each. It carried a crew of three -- pilot, navigator/bombardier and gunner/wireless operator and was armed with a forward firing 0.303 inch (7.7 mm) machine gun outboard of the port engine and a 0.303 inch machine gun in a semi-retracting dorsal turret firing to the rear. A 1,000-lb (454 kg) bombload was carried in the internal bay.
To achieve its relatively high speed, the Blenheim had a very small fuselage. 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 rather mediocre.
Specifications (Bristol Blenheim Mk IV)
Length: 42 ft 9 in (13 m)
Wingspan: 56 ft 4 in (17.17 m)
Height: 12 ft 10 in (3.91 m)
Wing area: 469 ft² (43.6 m²)
Empty weight: 9,790 lb (4,440 kg)
Loaded weight: 14,400 lb (6,530 kg)
Powerplant: 2× Bristol Mercury XV radial engine, 920 hp (690 kW) each
Maximum speed: 266 mph (231 knots, 428 km/h)
Range: 1,950 mi (1,690 nm, 3,140 km)
Service ceiling: 31,500 ft (9,600 m)
Rate of climb: 1,500 ft/min (7.6 m/s)
Wing loading: 30.7 lb/ft² (150 kg/m²)
Power/mass: 0.13 hp/lb (210 W/kg)
1× .303 in (7.7 mm) Vickers K machine gun in the nose
2× .303 in Browning machine guns in chin turret
2× .303 in Browning machine guns in dorsal turret
4× 250 lb (110 kg) bombs or
2× 500 lb (230 kg) bombs internally, and
8× 40 lb (18 kg) bombs externally