At any time the moral character, or capacity or general fitness for service, of any officer of the armed forces of the state may be determined by an efficiency board, consisting of three commissioned officers, senior in rank to the officer whose fitness for service is under investigation, and, if the findings of such board are unfavorable to such officer and are approved by the Governor, he shall be discharged. Commissions of officers may be vacated upon resignation, upon absence without leave for three months, upon the recommendation of an efficiency board or pursuant to sentence of a court-martial or, for officers of the National Guard and naval militia, for any other reason specified by laws or regulations of the United States pertaining thereto.