WASHINGTON — Defense Secretary Robert Gates pledged to oversee a disputed $35 billion tanker contract after congressional investigators on Wednesday detailed numerous mistakes the Air Force made in awarding the deal to Northrop Grumman and its European partner over Boeing Co.

In a redacted 67-page report, the Government Accountability Office found the Air Force failed to evaluate both refueling tanker proposals based on the same merits and repeatedly offered unfair preference to the team of Northrop and Airbus parent European Aeronautic Defence and Space Co. The extended report comes a week after the GAO upheld Boeing's contract protest and recommended the Air Force hold a new competition.

Gates was meeting Wednesday with Air Force and other Defense Department officials to discuss the tanker contract and determine how the GAO's recommendations could affect a decision to award a new deal, said Pentagon press secretary Geoff Morrell.