By the 1930s, Sikorsky was building transport aircraft and flying boats for Pan American Airways. His company, the Sikorsky Aviation Corporation in Bridgeport, Connecticut, became a subsidiary of the giant United Aircraft and Transport Corporation. Sikorsky and his aides occasionally sneaked helicopter models into the company wind tunnel. In 1931, he applied for a patent for a novel helicopter design that used a single main lifting rotor and a small vertical tail rotor to counteract torque. He later visited Europe to witness the flights of both Focke's Fa-61 and Bréguet's experimental gyroplane.

In 1938, as United Aircraft was closing down Sikorsky's company to cut costs, Sikorsky received permission to expand his helicopter research and to begin work on an experimental vehicle. In spring 1939, he designed the VS-300, which was built that summer. The VS-300 was constructed of an open welded tubular steel frame with three-wheel main landing gear. A three-bladed rotor with a diameter of 28 feet (8.5 meters) was mounted at the top. A single two-bladed vertical rotor was mounted at the tail. Both rotors were powered by a four-cylinder 75-horsepower (56-kilowatt) Lycoming air-cooled engine connected to the rotors through a truck transmission and a series of pulleys and belts. The main rotor could increase the pitch of the three blades collectively to change lift. Sikorsky also adopted cyclic control from Cierva's autogyro. 

Sikorsky made constant modifications to the craft in an attempt to reduce vibration and increase control. His mechanics began calling it "Igor's nightmare." By November 1939, the craft was making hops lasting a minute or two, but it was heavily damaged in December when a gust of wind toppled the machine and the rotor blades smashed against the ground.

cutaway of the XR-4

The machine made its first free flight on May 13, 1940. By this time, Sikorsky had added outriggers at the tail end and two additional tail rotors, and had switched to a more powerful 90-horsepower (67-kilowatt) Franklin engine. By mid-1940, the VS-300 was flying for 15 minutes at a time. In July, Captain Franklin Gregory, the project officer for the budding U.S. Army helicopter program, took the VS-300 for a test flight. He described the craft as having poor handling capabilities, saying that it flew like a bucking bronco. Gregory was an autogyro pilot, unaccustomed to the helicopter's unusual flight control system, which often required delicate hand movements. The U.S. Army Air Corps was very impressed, however, and awarded a contract to Sikorsky in December 1940 to build an experimental helicopter known as the XR-4, which was to be larger than the VS-300.

Sikorsky soon decided to eliminate the horizontal outriggers holding the tail rotors and mount the vertical tail rotor on a pylon added to the tail boom. This dramatically improved performance and soon the craft was flying forward stably at 70 miles per hour (113 kilometres per hour).

On January 14, 1942, the XR-4 was wheeled out and performed several short hops. By May, the craft was ready for delivery to Wright Field in Ohio. Sikorsky's test pilot flew the unusual aircraft on a series of short hops over five days from Connecticut to Ohio, and Sikorsky himself finished the trip. His test pilot navigated by reading highway signs or asking directions from surprised motorists.

The Army soon contracted for production to begin and also ordered a new, larger model. In April 1943, it requested another version. More than 400 Sikorsky helicopters were produced for the U.S. Army by the end of World War II. The R-4 saw service with the 1^st Air Commando unit in the China-Burma-India Theatre, where it performed some of the missions that would be later expanded during the Korean War and become common during the Vietnam War. Although they had no impact on the war, they clearly offered much promise.