The Pentagon on Friday announced that microfilm payroll records of large numbers of service members, including Bush, were ruined in 1996 and 1997 in a project to save large, brittle rolls of microfilm.
Bush's whereabouts during his service as a pilot in the Texas Air National Guard during the Vietnam War have become an election-year issue, with some Democrats accusing him of shirking his duty.
"The film just crumbled. We were attempting to improve the preservation," Defence Finance and Accounting Service spokesman Bryan Hubbard said.
"The film just crumbled. We were attempting to improve the preservation"
Defence Finance and Accounting Service spokesman
The destroyed files kept in Denver on deteriorating 2,000-foot rolls of microfilm covered three months of a period in 1972 and 1973 when Bush's claims of service with the guard in Alabama are in question.
The White House said it has already been shown that Bush performed his duties in the National Guard.
"We released all of the documents that are available. We made that clear at the time and they demonstrate that the president fulfilled his duties in the National Guard at the time. And there is nothing new in this respect," White House spokesman Claire Buchan said.
Last February, the White House released hundreds of pages of Bush's military records. But the records did not provide new evidence to place Bush in Alabama during the latter part of 1972, when Democrats say he was basically absent without leave.
Bush trained in 1969 to be a pilot. But in May 1972, he moved to Alabama to work on a political campaign and to perform his Guard service there for a year. But other Guard officers have said they had no recollection of ever seeing him there.