The Sikorsky H-19 Chickasaw (company model number S-55) was a multi-purpose helicopter used by the US Army and USAF.
It was also licensed-built by Westland Aircraft as the Westland Whirlwind in the United Kingdom.
Development of the H-19 was initiated privately by Sikorsky without government sponsorship. The helicopter was initially designed as a testbed for several novel design concepts intended to provide greater load-carrying ability in combination with easy maintenance.
The first customer was the USAF, which ordered five YH-19 aircraft for evaluation; the YH-19's first flight was on 10 November 1949.
This was followed by delivery of the first YH-19 to the USAF on 16 April 1950 and delivery of the first HO4S-1 helicopter to the US Navy on 31 August 1950.