The Pentagon announced on Wednesday that it would reopen bidding on a $35 billion contract for midair refueling tankers, allowing Boeing to continue its effort to wrest the business from a partnership of Northrop Grumman and the European parent of its rival Airbus.

Defense Secretary Robert M. Gates said that the tanker contract, which was won by the partners Northrop and the European Aeronautic Defense and Space Company in February, would not be awarded until the Pentagon reviewed the rival bids again.

The companies have battled fiercely to land the Air Force’s largest contract, with a potential value of $100 billion. The struggle has been caught in issues of national pride, trans-Atlantic relations and even presidential politics.

Mr. Gates’s action was a public rebuke to the Air Force, which had selected the offering of Northrop and EADS, the parent of Airbus. Shortly after the Air Force made its selection, the Government Accountability Office, acting on a protest from Boeing, said that the Air Force’s decision-making process was flawed and that the tanker contract should be reopened.