According to Gaijin, "The multi-staged beta test will allow tens of thousands of players to become better acquainted with the War Thunder universe, giving fans of the project a chance to contribute to the game’s development. Closed beta test servers will be open 24 hours a day, meaning that players who have been invited can join the game at any time. To start, participants will have access to more than 100 aircraft models and several different locations, with new content added continuously throughout the test period."
The developer also released a trailer commemorating the launch of the beta. Check it out in the video below.