The world's tallest military airplane is Hughes H-4 Hercules, with a height of 24.2m (79ft).
Howard Hughes's famous HK-1 Hercules is a cargo-type flying boat Originally conceived by Henry J. Kaiser, a steelmaker and builder of Liberty ships, the aircraft was designed and constructed by Hughes and his staff. The original proposal for the enormous, 400,000-pound wooden flying boat, with its spectacular 320-foot wingspan, came from the U.S. government in 1942. The goal was to build a cargo and troop carrier that did not require critical wartime materials; in other words, that substituted wood for metal. Throughout its construction, considerable controversy surrounded its funding. After a disgruntled U.S. Senator dubbed the HK-1 a "flying lumberyard," the "Spruce Goose" nickname was coined — Hughes despised the name!(www.theaviationzone.com)