According to the Museum’s website (which also provides technical specifications):

Gift of William Gallagher, Nancy C. Gallagher, T. Scott Gallagher and Nancy Gallagher Jones

Built in Danbury, Connecticut by Doman Helicopters Inc., this helicopter was FAA certificated and delivered to the U. S. Army in 1955. The design evolved in consultation with Army pilots from the ‘MASH’ medical evacuation activity after they flew a previous Doman experimental helicopter that had a very similar layout. It was the second of two aircraft, designated YH-31s, that were purchased by the Army and it was flown extensively at Fort Rucker and by the Navy at the Patuxent River Test Center. Then it was repurchased from the Navy and used by Doman in its commercial efforts.

It features the sealed, rigid and hingeless rotor system pioneered by Doman, and is powered by a 400 hp supercharged Lycoming engine that is cooled by exhaust ejectors rather than a fan. That feature increased its useful payload by about 800 pounds.

A total of only three craft were built, and this is the only surviving example.

Taken with a Sony RX-100 M3.