The aircraft was developed as Type 142, a civil airliner, in response to a challenge from Lord Rothermere to produce the fastest commercial aircraft in Europe. The Type 142 first flew in April 1935, and the Air Ministry, impressed by its performance, ordered a modified design as the Type 142M for the Royal Air Force (RAF) as a bomber. Deliveries of the newly named Blenheim to RAF squadrons commenced on 10 March 1937. A development of the Type 142M was the Type 149 which Bristol named the Bolingbroke, retrospectively changed by the Air Ministry to Blenheim Mk IV and the Type 142M to the Blenheim Mk I. Fairchild Canada built the Type 149 under licence as the Bolingbroke. Blenheims Mk I and the Mk IV were adapted as fighters by the addition of a gun pack of four Browning .303 machine guns in the bomb bay. The Mk IV was used as a long range fighter and as a maritime patrol aircraft; both aircraft were also used as bomber/gunnery trainers. The Blenheim was one of the first British aircraft with an all-metal stressed-skin construction, retractable landing gear, flaps, a powered gun turret and variable-pitch propellers. The Mk I was faster than most fighters in the late 1930s but the advance in development of monoplane fighters made all bombers more vulnerable particularly if flown in daylight, though it proved successful as a night fighter. The Blenheim was effective as a bomber but many were shot down. Both Blenheim types were used by overseas operators, being licence built in Yugoslavia and Finland. On 10 March 1937, production deliveries to the RAF formally started; 114 Squadron became the first squadron to receive the Blenheim. On 13 January 1938, the Blenheim entered service with No. 30 Squadron, the first overseas squadron to receive the type; in early 1939, the first Blenheims arrived in India. From July 1936 onwards, various additional orders were placed for the Blenheim Mk I, including multiple orders for the export market. By the end of 1936, 1,568 aircraft were on order. In order to meet the demand, secondary assembly lines were established at Chadderton by Avro and at Speke by Rootes Securities. The aircraft was built under licence by overseas countries, including Finland, who completed a total of 55 aircraft, and Yugoslavia, which completed 16 aircraft with a further 24 in advanced stages of completion when Germany invaded Yugoslavia. Other countries also procured the Blenheim, including Romania, Greece and Turkey. By September 1939, orders for the Blenheim had risen to 2,088 aircraft. Total production of the Blenheim Mk I in England was 1,351 aircraft prior to the end of the production run in 1939; production had been terminated in favour of more advanced variants. Formal 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). Only one Blenheim Mk II was completed, as flight tests revealed the increase in speed to be marginal and not warranting further development. Another modification resulted in the Blenheim Mk III, which lengthened the nose, dispensing with the "stepless cockpit" format of the Mk.I, introducing a true windscreen in front of the pilot, to provide more room for the bomb aimer. This required the nose to be "scooped out" in front of the pilot to maintain visibility during takeoff and landing. Both modifications were combined, along with a newer version of the Mercury engine with 905 hp (675 kW). The turret acquired a pair of Brownings in place of the original single Vickers K gun, creating the Blenheim Mk IV. Bristol Blenheim MkI, IV & V aircraft served with three RAAF article XV Squadrons (454, 456 & 459) with RAF serials. For more details of thedevelopment, design, operational history, and variants of the Blenheim, click here. Blenheim Mk I Blenheim Mk IV (Note: The Finnish Air Force used a blue swastika as its emblem until 2000.)
