Helicopter design started in the 1930s. During World War II, the Germans developed early military helicopters (although they saw limited service and their factories were heavily bombed by the Allies). Helicopters were used in limited amounts by the US in the Korean War, but it wasn’t until the Vietnam War of the early 1960s that military helicopters really came into their own. Some of these Vietnam-era US helicopters remain in use today by the US military and militaries around the world. The Mil V-12: The Story Of The Largest Helicopter Ever Built built in the 1968s (although the design proved unsuccessful).