The M.39B Libellula (from Libellulidae, a taxonomic family of dragonflies) was a Second World War tandem wing experimental aircraft built by Miles Aircraft, designed to give the pilot the best view possible for landing on aircraft carriers.
To prove the concept Miles designed and built a 5/8th scale version, the M.39B, which flew on 22 July 1943, showing no "undesirable handling" characteristics. It coincided with interest by the authorities in unorthodox designs for large aircraft. The rear wing was mounted higher than the forward one to avoid downwash and give ground clearance for the propellers. The M39 design had inboard flaps and outboard ailerons on the rear wing and the front wing had an auxiliary aerofoil/flap/elevator device, which could vary the wing area without changing lift coefficient.[5]
Well, mixture of factors. One is that WW2 was winding up, and they didn't have a strong need for it anymore - another that the plane didn't perform as well as they hoped, and that jets were starting to be recognized as the next real 'step' in aircraft development, and jets really need to be made from scratch, not just slapped onto pre-existing designs. So eventually they just scrapped the whole thing.
The Wright brothers' first flight was 40 years prior. They were trying all sorts of crazy stuff and relaxed, for the most part, in the 60s. They invented the helicopter, an aircraft that stays aloft through sheer violence alone, in '39.