World War II was a catalyst for aircraft development. Initially, the piston aircraft of World War II, though they often flew at very high altitudes, were not pressurized and relied on oxygen masks. This became impractical with the development of larger bombers where crew were required to move about the cabin and this led to the first bomber with cabin pressurization (though restricted to crew areas), the Boeing B-29 Superfortress. The control system for this was designed by Garrett AiResearch Manufacturing Company, drawing in part on licensing of patents held by Boeing for the Stratoliner.