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. The White House said the document had been in Bush's personnel file and that it had been found by the Pentagon.
Earlier Wednesday, the White House said Bush never was disciplined while serving in the Texas Air National Guard, never failed a physical and never asked his father or family friends for help to get him into the Guard.
The White House assertions came in response to a dozen questions submitted by AP in light of new records detailing Bush's Guard service and allegations that have surfaced this election season.
The president's critics say he got help getting into the Guard and was treated with kid gloves once he became a guardsman. Democrats question why Bush was never punished for skipping a required medical examination or missing drills for six months in 1972.
Bush has maintained he fulfilled all of his National Guard requirements and served honorably.
AP asked whether Bush ever participated in a disciplinary process during his Guard service, whether he ever received a critical report or was ever present for a conversation in which his performance, conduct or physical condition were raised by a superior officer.
"No and this is clear from the president's records, which have been made public," the White House said in an e-mail response.
The Texas Air National Guard stripped Bush of his pilot status in August 1972 for failing to take the annual medical exam required of all pilots. Former Air National Guard officials say it was rare for a pilot to skip his physical exam.
"No," the White House replied when asked whether Bush ever failed a medical exam in the Guard or had a medical problem that would have temporarily or permanently disqualified him from flying.
The White House said, "The president did not ask his father or family friends for assistance" in getting into the Texas Air National Guard during the Vietnam War.
The AP filed lawsuits in federal court and state court in Texas seeking any additional records of Bush's Guard service after the White House said in February that everything had been released.
Documents released since then include Bush's official flight logs as a fighter pilot, showing he spent more than 300 hours in military jets but had shifted to a two-seat training jet several times in his final weeks as a pilot in 1972. Bush spent 40 percent of his flight time in training jets or simulators during the first four months of 1972, double the percentage for the previous five months.
"There could be many reasons why an individual pilot would fly in a training plane, including availability of the planes," the White House said in its written response.
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.
The White House said Bush skipped the exam because he expected a transfer to an Alabama unit which did not have the F-102A jets Bush was trained to fly. Bush has said he went to Alabama to work on the unsuccessful U.S. Senate campaign of Winton Blount, a family friend.
"The president was transferring to Alabama to perform equivalent duty in a non-flying capacity, making a flight physical unnecessary," the White House said.
The White House did not answer whether Bush disobeyed a direct order to take the exam. Bush won final approval to train for three months with an Alabama unit a month after he had already been suspended as a pilot.