The R-4 was the world's first large-scale mass-produced helicopter and the first helicopter to see service with the USAAF and others (USN, RAF, etc.). This example was TOC in April 1944 with the USAAF and at the end of WWII became N92820 with United Aircraft Corp and used for testing by Sikorsky. This registration was cancelled in October 1955 and the helicopter had become part of NEAM by August 1994. This isn't the nicest of photos but it shows nicely the difference between the conventional drive-shaft driven rotor on the R-4B and the hingeless rotor of the Doman YH-31 in the foreground.