D.C. Code § 49-402

In addition to the persons exempted from enrollment in the militia by the general laws of the United States, the following persons shall also be exempted from enrollment in the militia of the District of Columbia, namely: Officers of the government of the District of Columbia; judges and officers of the courts of the District of Columbia; officers who have held commissions in the regular or volunteer Army, Navy, or Air Force of the United States; officers who have served for a period of 5 years in the militia of the District of Columbia or of any state of the United States; ministers of the gospel; practicing physicians; and conductors and engine-drivers of railroad trains.