There shall be under the adjutant general one assistant adjutant general who shall be director of the army national guard and one assistant adjutant general who shall be director of the air national guard, both of whom shall have at least the rank of brigadier general. Under the adjutant general, there shall also be a staff for the national guard. The staff shall consist of such officers as shall be designated in permanent orders and shall perform such functions and duties as the adjutant general may prescribe.