The Air Force has made another potentially serious mistake in its effort to buy $35 billion of aerial tankers, by accidentally sending each bidder sensitive data about the other’s proposal, Pentagon officials said Saturday.

Officials said both companies, Boeing and the European Aeronautic Defense and Space Company, promptly returned the information, and the Air Force plans to continue the bidding.

But industry consultants said the mistake could provide the loser with grounds to protest the contract, delaying a decade-long push to replace refueling planes from the Eisenhower era.

“This seals the deal that this contract award will not be the last word,” said Richard L. Aboulafia, an analyst at the Teal Group in Fairfax, Va.