In 1935, the British Air Ministry issued Specification G.24/35 to procure a coastal reconnaissance/light bomber to replace the Avro Anson. Bristol proposed the Type 149, based on its Blenheim Mk I, with Bristol Aquila engines to give greater range. While the Air Ministry rejected this proposal, a Blenheim Mk I, retaining its Mercury VIII engines, was converted as a Type 149 (Blenheim Mk III) for the general reconnaissance role. The nose was lengthened to provide more room for the bombardier, with the upper left surface of the nose being scooped out to maintain pilot visibility during takeoff and landing.
The longer range also fulfilled a Canadian requirement for a maritime patrol aircraft. Consequently, Fairchild Aircraft Ltd. (Canada) of Quebec started production of the Blenheim Mk IV as the Bolingbroke (the originally intended name for the Blenheim IV). This type was nicknamed the "Bolly". After a small run of aircraft constructed to British specifications, as the Bolingbroke Mk I, Fairchild switched production to the Bolingbroke Mk IV with Canadian and American instruments and equipment. These versions also included anti-icing boots and a dinghy. One of the early Mk IV variants was the Bolingbroke Mk IVW which was powered by two 825 hp (615 kW) Pratt & Whitney SB4G Twin Wasp Junior engines. Incapable of maintaining altitude on one engine, the normal bomb load was reduced to 500 pounds on these aircraft to compensate for the low engine power. The most-produced variant was the Bolingbroke Mk IVTtrainer, of which 457 were completed. A total of 626 Bolingbrokes were produced.
Most of the 151 Mk IVs built served in their intended role as patrol bombers on the Atlantic and Pacific coasts of Canada between 1940 and 1944. Two squadrons of these aircraft also served in Alaska during the Aleutians campaign. The Mk IVT trainers saw extensive use in the British Commonwealth Air Training Plan (BCATP).
Twin-engine maritime patrol bomber aircraft, powered by two Bristol Mercury VIII radial piston engines, with British equipment. 18 built.
Bolingbroke Mk II
Conversion of fifth Mk I with US equipment - prototype of Mk IV.
Bolingbroke Mk III
Floatplane conversion of sixteenth Bolingbroke Mk I, with two Edo floats.
Bolingbroke Mk IV
Twin-engine maritime patrol bomber aircraft, equipped with anti-icing boots and a dinghy, also fitted with American and Canadian instruments and equipment, powered by two Bristol Mercury XV radial piston engines, 134 built.
Bolingbroke Mk IVW
Sub-version of Mk IV powered by two 825 hp (615 kW) Pratt & Whitney SB4G Twin Wasp Junior radial piston engines as contingency against possible shortages of Mercury engines. The Mk IVW's performance was below that of the Mk IV and the supply of the British engines was maintained so production returned to the Mk IV after only 15 aircraft were built.
Multi-purpose trainer aircraft. A total of 350 built powered by Mercury XV engines, followed by a further 107 powered by the low-octane fuel Mercury XX* engine, giving a total of 457 built, with a further 51 cancelled. 6 Mk IVT were converted to dual controls. A further 89 were converted to Mk IVTT Target Tug with the addition of winching gear in the rear cabin and target drogue storage in the bomb bay.
IVT 10201 This was restored to airworthy condition by the Aircraft Restoration Company at Imperial War Museum Duxford as G-BPIV. It is currently being fitted with a Blenheim Mk I nose which had previously been converted to a car.