The M.39B was received by the Royal Aircraft Establishment (RAE) for evaluations and problems persisted from the get-go. A new undercarriage had to be fitted and several accidents kept the machine in constant repair and grounded from useful flying. Overall, handling characteristics were deemed as quite normal for the aircraft despite its unusual arrangement. Landing and take-off required some changes to the usual routine but this was forgivable. Stalling and recovery was as expected.
Back in 1941, the RAF issued a new specification (B.11/41) calling for a medium-class, medium-to-high-altitude, high performance bomber. The Miles response was to offer its M.39 Project, a revised version of M.39B featuring a crew of three in a pressurized cockpit. Power would be from 2 x Rolls-Royce Merlin 61 series inline engines or 3 x Power Jets W.2/500 gas-powered turbine engines. The wingspan was drastically increased to 37.5 feet at the frontal wing and 55.8 feet at the rear pairing. Overall length grew to become 35.9 feet and maximum weight ballooned to 26,750 pounds. Proposed cruising speed was 360 miles per hour and armament was to be 2 x 20mm cannons in the wing roots (one gun per root) with up to 6,000 pounds in bombs carried through an internal bomb bay found at the center of the fuselage.
The B.11/41 specification was eventually fulfilled by the de Havilland DH.99 "Vampire". The Hawker P.1005 was also in the running for a time.
The prototype M.39 high-speed bomber prototype was ordered but never constructed to an issued November 1943 contract. The RAF bomber requirement was fulfilled elsewhere and the M.39 Project terminated along with the M.39B data-collecting aircraft. While surviving the war, M.39B lasted only until 1948 when it was scrapped.
One final proposal related to the Libellula endeavor was the M.63. This aircraft was to instead feature three clustered jet engines in the rear fuselage, doing away with the original design's wing-mounted, prop-driven engine nacelles (as well as the central dorsal fin featured in the M.39B). The result was to be a more streamlined variation retaining all of the capabilities of the M.39 high-speed bomber form and furnished as mailplane - though even this Miles proposal fell to naught.
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