10 U.S.C. § 12204

(a) No person may be appointed as a Reserve in a commissioned grade above major or lieutenant commander, unless— (1) he was formerly a commissioned officer of an armed force; or (2) such an appointment is recommended by a board of officers convened by the Secretary concerned. (b) This section does not apply to adjutants general and assistant adjutants general of the several States, Puerto Rico, and the District of Columbia.