This is a Significant Operational milestone.
The McDonnell XV-1 was an early compound helicopter (compounded with thrust and lift). A tethered hover flight had taken place earlier on Feb 11 1954.
Note: See Discussion forum topic on this milestone.
The aircraft became the first compound helicopter to exceed 200 mph in level fight. A very thorough review of the development of the XV-1 can be found in,
"An Overview of Autogyros and The McDonnell XV-1 Convertiplane", Franklin D. Harris, NASA/CR-2003-212799, October 2003
Source: AHS Calendar July 2013