This is a Significant Technology milestone.
This was the first time that an aircraft with tilting rotor shafts completed a full transition. The transition covered the full regime from vertical prop rotors in low speed VTOL flight to the prop-rotors being fully tilted forward to 90 degrees in cruise. In this latter condition the entire aircraft weight was carried by the fixed wings. The pilot was Bell Helicopter test pilot Bill Quinlan.
This was a critical step in the development of tilting rotor technology.
Source: VFS 2019 VTOL History Calendar