An experimental aircraft designed for the Royal Aircraft Establishment, or the Ministry of Defense as it is known today, was the Miles M.39B. With its dual main wings the M39 had been aptly
named the Luebllula, a taxonomic family of dragonflies to which it bears some resemblance. The craft was intended to be a fast bomber capable of operating from an aircraft carrier. As such the 
design of the wings allowed it to be safely stored without the need for fold-able wing. This was achieved by shortening the main wings, adding a second wing to achieve the desired lift.
This aircraft is essentially a redesigned M.35. Also manufactured by Miles, a prototype M.35 had been constructed in record time and commenced flight tests on July 22, 1943. Unfortunately, the M.35 suffered from many aerodynamic instabilities. The aircraft was then redesigned to ultimately end up as the M.39 with Miles acting on a specification issued by the Royal Air Force for a high altitude, high performance bomber.
The design of the aircraft called for three Power Jet W.2/500 turbojet engines. However, since the engines were still under development, Miles opted for two prototype Rolls-Royce Merlin 60 engines for the prototype aircraft. These engines had a two speed supercharger, capable of supplying the massive V-12 engine with ample air. The engines themselves were capable of putting out a massive 1390 horsepower at 3000 rpm, ideal for a fast bomber.
After having a 5/8th scale design, named the M.39B successfully complete flight testing, the design was
further refined and finally handed over to the Royal Aircraft Establishment in 1944 carrying the serial number SR392. An iconic feature of this design had been the addition of a third dorsal wing aft of the fuselage, giving the aircraft its signature outlines. The second main wing that came from the original design was slightly reworked. This wing had the added benefit of providing extra lift to the aircraft.
Specifications and Design
The final Libellula prototype was powered by two de Havilland Gipsy Major IC inline piston engines producing a total of 140 horsepower each. Originally designed to be turbojet powered, the Gipsy engines afforded dismal performance to the plane, only allowing it to fly with a maximum speed of 102 mph. Due to the low wing loading and large surface area of the wings the plane had excellent range.
Ultimately, the plane was to be manned by a crew of three. However, the scaled down version only had the pilot. It measured in at a total of only 22 ft. with a height of 9 ft.
During the testing phase, no armament had been fitted. The production model of the M39 would have been outfitted with two 20 mm cannon sitting in the wing roots. This, together with the planned ability of carrying 6000 pounds of bombs would have likely had it achieve some successes on the battlefield. However, the slow speed of the plane raised doubts about its abilities in battle.
During 1941, a new specification had been issued by the RAF. This called for a high altitude bomber also capable of high performance. The M.39B’s design was then revised to meet these specification. The first and most important change had been that the cockpit was redesigned to be a pressurized cockpit, allowing the crew to survive high altitude flight. The engines specification had also been revised to a Merlin 61. The Merlin 61 was slightly more powerful than the Merlin 60, measuring a power output of 1565 horsepower. It had also been developed to a production model whereas the Merlin 60 had only been a prototype.
Testing program
During flight trials, a few accidents left the undercarriage severely damaged effectively grounding the aircraft for most of its life. It is also during this testing phase that another plane, the Havilland DH.99 made great success fulfilling the specification issued by the RAF.
Most of the handling characteristics of the M.39B proved above average, with landings and take-offs a maneuver a bit out of the ordinary, but still easily manageable.
Ultimately the contract had been afforded for the construction of the M.39 prototype in 1943. However with the success that the DH.99 had been achieving the program was eventually cancelled. This saw both the M.39 and M.39B being terminated and ultimately scraped in 1948, reduced to scrap.
