Yahoo! News – Document: Bush Leaves Military Service

The White House on Wednesday night produced a November 1974 document bearing Bush’s signature from Cambridge, Mass., where he was attending Harvard Business School, saying he had decided not to continue as a member of the military reserve.
The document, signed a year after Bush left the Texas Air National Guard, said he was leaving the military because of “inadequate time to fulfill possible future commitments.”
White House spokesman Scott McClellan said the resignation was found in connection with a lawsuit brought by The Associated Press. The White House said the document had been in Bush’s personnel file and that it had been found by the Pentagon

Bush needed to take a physical exam by the end of July 1972 to keep flying. But he skipped the physical and his commanders grounded him in August 1972. Bush never flew for the military again.