The M.39 "Libellula" was conceived of by the Miles Aircraft concern during the early 1940s as World War 2 (1939-1945) raged on. The initial concept was to reduce the complexity of landing aircraft on carrier decks where vision was largely obscured by the forward-mounted engine and large wing mainplanes. A new approach was undertaken which intended to reduce the risks involved and this became a "tandem-wing" arrangement that ultimately covered a series of concepts for Miles during the war. However, none of the concepts were realized through serial production contracts though M.39B was built and flown as a prototype.



Collectively, the designs making up this Miles endeavor were named "Libellula", the taxonomic family name of the dragonfly (no doubt in reference to the Miles' aircraft use of twin wing mainplanes).



The original work on the subject was through the M.35 offering which was more or less a technology demonstrator for a single-seat naval fighter. It was developed along the lines of a private venture by Miles and its tandem-wing arrangement was used to reassess the Center of Gravity (CoG) seen in traditionally-arranged military aircraft (mainplane at front with tailplanes at rear). The tandem-wing approach allowed for a greater CoG range between the two wing mainplanes in play and could, theoretically, produce some inherent benefits - a shortened fuselage and more contained wingspan for onboard carrier storage (as well as reduced weight), less complex wings as wing-folding mechanisms would not have to be used, and improved control from the "doubling up" of control surfaces along the wing pairing.



Covering the short span of six weeks, Miles engineers returned with the M.35. This aircraft was powered by a single de Havilland Gipsy Major air-cooled inline engine of 130 horsepower and featured a wingspan of 20 feet at the frontal mainplane and 20.5 feet at the rear mainplane. Overall length was 20.3 feet and maximum weight reached 2,000 pounds. The cockpit was held at the extreme forward of the fuselage and offered excellent vision. The frontal wing mainplane was shoulder-mounted and of a straight design whilst the rear mainplane was low-mounted and featured sweepback. Vertical fins were affixed to the rear mainplane's tips. The engine was buried in the rear portion of the fuselage and arranged in a "pusher" configuration. A pseudo-tricycle undercarriage was fitted though a forth leg was situated at the extreme rear to protect the aft sections of the aircraft. In testing the engine proved temperamental and longitudinal control was off. Despite this, the design proved to shed some light on the unorthodox arrangement which would help with future work in the field.



The work was furthered into a new design which was of 5/8th scale and designated as M.39B. The aircraft would serve as a single-seat demonstrator and be powered by a pair of de Havilland Gipsy Major IC air-cooled inline engines of 130 horsepower each - these showcased as outboard nacelles hung under each rear wing mainplane. The frontal mainplane was now fitted low against the sides of the cockpit. The fuselage was given a finely contoured shape from nose to tail. A tricycle undercarriage - now in the traditional sense -was fitted.



Since the M.39B was of a larger project scope, its dimensions was appropriately increased. The wingspans now reached 25 feet at the front set and 37.5 feet at the rear. Overall length was 22.3 feet and maximum weight increased to 3,200 pounds. As built and tested, M.39B was capable of speeds reaching 166 miles per hour from its twin-engine fit and tandem-wing approach. First flight for the M.39B came on July 22nd, 1943. A dorsal fin was later fitted at the fuselage aft for additional control to complete the M.39B's iconic three-finned appearance.



Up until September of 1943, all of the work on the two aircraft, M.35 and M.39B, was as a private venture. From then on, the British Air Ministry decided to offer a formal development contract to Miles. Shortly thereafter, Miles attempted to elicit interest from the United States Army Air Forces (USAAF) who were already considering the homegrown Curtis XP-55 "Ascender" which featured a similar tandem-wing, pusher-engine arrangement and slated as a fighter. The USAAF response was of general disinterest in the Miles product for the XP-55 showcased problems all its own - particularly with stalls and recovery. Only flight-testing was ever reached with the American program and three aircraft built in all.