There are no guarantees, unfortunately, but Gaijin's announcement says that "the more you complete the higher your chances are." Today's mission involves destroying 60 ground units in War Thunder's arcade mode or 30 ground units in the title's historical battles mode or full real battles mode.
War Thunder currently features instanced World War II-era aerial combat. The dev team is in the process of adding ground and eventually naval combat as well. The ground forces closed beta is scheduled to start "before December 4th."