The B Mk IX was a high altitude development of the Mosquito B Mk IV. Development work on the original prototype Mosquito began in 1942, when it was fitted with Merlin 61 two stage supercharged engines and four bladed propellers. In this configuration it reached a height of 40,000 feet in June 1942, the highest then achieved by a Mosquito. With three bladed propellers it then produced a top speed of 432 mph, 50 mph faster than the Mk IV. Later tests with the Merlin 77 engine saw the prototype reach a speed of 437 mph at 29,000 feet, a speed that no other Mosquito would ever beat.
The production Mk IX used the Merlin 72 engine. This gave a maximum of 1,680hp and still provided 1,505hp at 21,000 feet, an altitude at which many engines began to lose power. The change of engine required a change in the engine nacelles, which now needed air intakes for the engine supercharger.
The B Mk IX was the first of the bomber Mosquitoes to use a “universal” wing that could carry either fuel drop tanks or two extra bombs, giving it a potential bomb load of 3,000lbs.
The first of 54 production Mk IXs flew in March 1943. The type reached No. 109 squadron on 21 April 1943. When equipped with electronic bombing aids, the Mosquito B Mk IX could bomb from a height of 30,000 feet, three thousand feet above the service ceiling of the Mk IV!
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