Bristol Blenheim – Introduction and Development

The Bristol Blenheim was a light bomber, that when first designed was one of the fastest military aircraft in the world. Sadly, by the time war broke out in 1939 it had been leapfrogged by more modern aircraft, and the Blenheim would turn out to be slow and vulnerable to modern fighters. Despite this, over six thousand Blenheims and Bolingbrokes would be produced, and the aircraft would serve in just about every major theatre of the Second World War.

The Blenheim evolved out of a planned light passenger plane, the Bristol Type 135. This was under development between 1933 and 1935. In 1934 the aircraft attracted the attention of Lord Rothermere, the owner of the Daily Mail, who had been a long term supporter of British aviation. He needed an aircraft with longer range. Work on this aircraft, the Type 142, began in April 1934. At the same time Bristol began work on a possible military variant, with the company designation Type 143, although it would be the Type 142 that would become the Blenheim.

The Type 142, powered by two 640hp Mercury VI engines, first flew on 12 April 1935. It was an astonishingly fast aircraft, with a top speed of over 300mph. It massively outclassed any fighter aircraft then in service. When the Air Ministry asked to borrow the aircraft for testing, Lord Rothermere went one step better and donated it.

Tests at Martlesham Heath were encouraging. Even with a full load, the aircraft still had a top speed of 285mph. The Air Ministry issued specification B.28/35 in August 1935. This specification covered the conversion of the Type 142 into a three man light bomber, the type 142M.

Bristol Blenheim Front View
Bristol Blenheim Mk I
in Greece

The design for this was complete by September 1935, when the Air Ministry placed an order for 150 Blenheim Mk Is. The biggest change between the Type 142 and the Blenheim was in the wing position, which was moved from the low wing position of the passenger plane, to a mid wing position. This allowed the bomb bay to be positioned below the central wing spar. Other changes included the creation of a navigator/ bomb aimer position below the pilot’s position, and the fitting of armament. Defensive armament consisted of a single .303 Browning machine gun in the leading edge of the port wing, and a Bristol turret, armed with one .303 Lewis or Vickers K gun.

The fate of the Blenheim was sealed by its slow development. The first prototype was delivered to the RAF in June 1936. Full production was authorised in December 1936, and deliveries began in March 1937, four years after work had begun on the original Type 135. The Blenheim was already too slow. The Messerschmitt Bf-109B also entered service, with a top speed of 292mph, compared to the 280mph of the Blenheim. While successive versions of the Bf-109 would get faster, the Blenheim would actually lose speed over its three main versions.

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How to cite this article: Rickard, J (26 June 2007), Bristol Blenheim Introduction and Development,

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