D.C. Code § 49-309
The Commanding General is authorized to prescribe a system of examination to be conducted at such times anterior to the accruing of the right to promotion as may be best for the interest of the service. If any officer fails to appear for examination within 30 days after notification to so appear or fails to pass a satisfactory examination and is reported unfit for promotion, the officer next below him in rank, having passed said examination, shall receive the promotion; and provided, that should the officer fail in his physical examination and be found incapacitated for service by reason of physical disability contracted in the line of duty he shall be retired with the rank to which his seniority entitled him to be promoted; but if he should fail for any other reason he shall be suspended from promotion for 90 days, when he shall be reexamined, and in case of failure on such reexamination he shall be honorably discharged.
D.C. Code § 49-309