When it first flew on 12th April 1935, the civilian Bristol Type 142 proved to be faster than any fighter then in service with the RAF and this quickly led to an Air Ministry specification for a bomber version. The resulting Type 142M carried a crew of three – pilot, navigator/bomb aimer and gunner – and was powered by two Bristol Mercury VIII radial engines each of 860hp. Armament comprised a forward firing .303 in Browning machine gun and either one or two .303 in machine guns mounted in a dorsal turret firing to the rear. The internal bomb bay was capable of carrying a 1,000 lb load.
The aircraft had a very small fuselage cross-section. The pilot’s cockpit area was so cramped that the control column obscured the flight instruments and such essential items as the propeller pitch control were positioned behind the pilot to be operated by feel alone. The bomb bay doors were secured in the closed position by cords and opened under the weight of released bombs – it was quite impossible to forecast how long it would take for the doors to open and the bombs to fall and hence bombing accuracy was poor.
The Type 142M soon became known as the Blenheim and entered RAF service with No114 Squadron in March 1937. Various modifications were incorporated to provide enhanced performance but by the outbreak of war in September 1939, the aircraft was essentially obsolescent, largely due to its light defensive armament and susceptibility to flak. Nevertheless, it was used extensively during the Battle of Britain albeit incurring heavy losses; indeed a casualty rate approaching 100% was experienced on some raids as exemplified on 13th August 1940 when, during a 12 aircraft operation against Aalborg in Denmark, one machine returned early and the remaining eleven were shot down.
The Blenheim achieved some success as a night fighter equipped with a Mk III AI radar and fuselage-mounted gun pack carrying 4 x .303 in Brownings. A special Fighter Interception Unit was formed at RAF Tangmere in April 1940 in order to train crews in intercept techniques and it was on the night of 22nd/23rd July that one of its aircraft achieved the first ever AI radar assisted ‘kill’. Whilst on patrol south of Selsey Bill, ground radar vectored the pilot on to an enemy formation whereupon the AI operator acquired a ‘blip’ and controlled an intercept through to visual range. The target, a Dornier 17, was duly dispatched and crashed into the sea an estimated 5 miles south of Bognor Regis.
The Blenheim was operated by the air arms of 12 other countries. A version known as the Bolingbroke was built in Canada and served with the RCAF mainly in training roles. A total of 4,422 machines were built, a number of which survive as static display aircraft at museums around the world. The one remaining flying example in recent years crashed in 2003 and is now being restored to airworthiness by the Aircraft Restoration Company, Duxford.