This was a program sponsored by the Office of Naval Research, using a Kaman HTK-1 as the test vehicle (designated HTK-1K)

A video of the aircraft in flight can be seen by selecting the link below,

Kaman HTK-1K - the first unmanned helicopter, by sobchakvideos, YouTube, accessed Mar 29 2017

Source: "The Kaman Aircraft Corporation", Charles Kirchner, Journal of the American Helicopter Society, Volume 1, Number 1, January 1956, Page 54

and "Kaman's Robot", FLIGHT Staff, FLIGHT, 19 October 1956, Page 640

Note: The exact day for this milestone is unknown