The Blenheim Mk IV was first developed as a general reconnaissance aircraft – an all purpose maritime wartime aircraft, to fill the gap between the Avro Anson and the upcoming Bristol Beaufort. In the event, that gap was filled by the Lockheed Hudson, and the Blenheim Mk IV would serve in the same bomber role as the Mk I. The Mk IV was first developed as the Bristol Type 149 “Bolingbroke”, and was actually produced under that name in Canada.
Initial work was centred on solving two problems with the Mk I. First, the short nose had not been popular, especially with navigators, who had a rather cramped position below the pilot. Second, the range of the aircraft needed increasing to cope with long patrols over water.
The range problem was solved by added a 94 gallon fuel tank in the outer wing. This increased the range of the aircraft by around 500 miles, but did mean that the aircraft was too heavy to land when fully loaded. A fuel jettison system was added to allow the pilot to bring down the weight of the aircraft in an emergency.
The nose needed rather more work. The first design, mocked up in October 1936, involved extending the nose of the Mk I by three feet, giving the navigator/ bomb aimer much more space. Tests in October 1937 revealed problems with this - the windscreen was too far from the pilot, causing problems with glare, reflections and distorted view.
The solution adopted was to produce an even longer nose, but to lower the top of the navigator/ bomb aimers’ compartment, and move the pilot’s windscreen back towards him. Finally, the starboard roof of the nose was lowered even further, giving the Blenheim an asymmetrical appearance from the front.
Early Mk IVs carried the defensive armament at the Mk I – a single forward firing .303in Browning machine gun in the port wing, and a single .303 in Vickers machine gun in the dorsal turret.
However, combat experience soon revealed that the aircraft was badly under-gunned, and a variety of improvements were carried out. The turret was upgraded to carry two Vickers or Browning machine guns. A free mounted Vickers “K” gun was added to the nose. Finally, rear-firing ventral turrets were fixed under the nose, aimed via a periscope by the navigator. These were of the Frazer Nash FN 54 or 54 A types, carrying one or two .303s.
The Mk IV proved to be slightly slower than the Mk I. It had a much lower service ceiling, down by 5,000 feet to 22,000 ft. Bomb load remained the same, at 1000 lb, normally made up of four 250lb or two 500lb bombs.
Production of the Mk IV was much delayed. The first orders for the type were placed in 1936, but the first deliveries did not start until early 1939. After that production accelerated, with Avro and Rootes the main suppliers. On 3 September 1939, the R.A.F. had 168 Blenheim IVs equipping ten squadrons.
As with the Mk I, there was a fighter variant of the Mk IV, created by placed a gun pack, carrying four .303in machine guns, in the bomb bay. This variant served as a night fighter with Fighter Command, and with Coastal Command.
Introduction - Combat Record - Mk I - Mk IV - Mk V - Bisley - Bolingbroke - Stats - Picture Gallery