The Curtiss-Wright X-100 was built to demonstrate the propeller 'radial force' concept as a means of providing lift.The first tethered hover was on Apr 20, 1959.

The aircraft made its first transition beween vertical and horizontal flight in April 1960.

Source: "Curtiss-Wright X-100 Tilt-Prop VTOL", Ken Bartie, VFS web page, Last accessed Feb 25, 2020