While balsa wood is certainly light it doesn't have much strength. Hollywood uses balsa all the time for making prop structures that stuntmen can break through easily without getting hurt. When Howard Hughes was charged with making the worlds largest troop transporting airplane he needed a strong but light weight material and metal was scarce and too heavy anyway. It was decided to build it out of wood and the wood chosen for it's best strength to weight ratio was Sitka spruce. This was no real discovery though as guitar and stringed instrument makers had been building instruments with spruce tops for some of the same reasons long before. The Spruce Goose, as Howard's plane was dubbed, actually flew one test flight, and has been in a hangar ever since!