Designed by Igor Sikorsky, the R-4 was the first helicopter in use outside of Germany and built in larger numbers than any other of its time. The R-4 had a unique design that has become the standard for most helicopters since: a single lift rotor and a long tail which had a single anti-torque rotor. The nose cabin had two seats in side-by-side arrangement, accessible through side doors and with optional dual controls. Fixed landing gear were typically installed, although some were fitted with inflatable rubber pontoons for naval operations. The R-4 was extremely versatile, serving with both the US Army and US Navy and successfully undertook dangerous rescue missions during World War II in difficult terrain, such as the China-Burma-India theater and the South Pacific. Around one-third of all R-4s built served with the RAF, where they were given the name Hoverfly (never officially adopted in the US). After the war, R-4s were replaced by other Sikorsky designs such as the R-5 and R-6, both of which were based on this design.

First flight of the prototype XR-4 took place on 13 January 1942, and was based on the earlier VS-300. Following a couple of other experimental versions, the production R-4 was introduced that same year with a more powerful engine. A number of these were transferred to the US Navy where they were known as the HSN-1, while others were provided to the UK where they were known as the Hoverfly.