The US Army Air Corps accepts it's first helicopter (Sikorsky XR-4)
Saturday, May 30, 1942
Colonel Frank Gregory headed the evaluation team. The XR-4 was the world's first mass produced helicopter and the first successful production rotorcraft with a single rotor configuration.
The aircraft had been delivered (by Ralph Alex, project officer on the helicopter) to Wright Field on May 17 1942 after a cross-country flight from Stratford, Connecticut.
Source: Smithsonian National Air and Space Museum | Collections | Vought-Sikorsky XR-4C | Long Description, Accessed April 16, 2016