This is a Notable Operational milestone.
The milestone represents the first hover flight of the NACA/U.S. Army/Bell XV-3 Tilt Rotor. Pilot was Floyd Carlson (see Bell company history web page). The aircraft had been unveiled on Feb 10, 1955 at the Bell plant in Hurst, TX.
The type was the first tilt rotor aircraft aircraft to fully complete a conversion from rotor borne to wing borne flight. This feat was actually accomplished by the Ship 2 prototype, which had a different configuration.
See AHS Calendar December 2013 for more details.
An example of the XV-3 can be seen at the National Museum of the United States Air Force.