Germany was one of the early leaders in developing helicopters in WWII (e.g., its limited production Fa 223 Drache entered service in 1941). But the United States was not far behind and by January 1943, the US Army had introduced its first successful helicopter (the R-4B Hoverfly) into service. While helicopters would only see limited use in WWII, they would see much more extensive use in the Korean War before coming into their own in the Vietnam War. Here are five facts about the early US development of helicopters in World War II.