By Jim Wolf

WASHINGTON (Reuters) - Airbus parent EADS <EAD.PA> is sticking to plans to boycott a potential $50 billion competition to build a U.S. Air Force refueling fleet absent major changes in the way the winner would be picked.

"This is not a negotiating ploy," Sean O'Keefe, chief executive of EADS' North America unit, told reporters at a briefing on Friday.

EADS has joined Northrop Grumman Corp <NOC.N> to compete against Boeing Co <BA.N> for a rematch to build an initial 179 aircraft that refuel others in flight.

Northrop, which would be the team's prime contractor, said on December 1 it had concluded that a draft request for proposals (RFP) issued by the Air Force favored a smaller tanker of a type Boeing could offer based on its 767 wide-body model.

Northrop Chief Executive Ron Sugar denied the company was trying to dictate requirements for the Air Force, as some Boeing supporters have suggested, but said there should be no mistaking the team's resolve.

"Nobody should make a mistake. We cannot bid based on this current RFP," Sugar told Reuters in a telephone interview.

He said there was "constructive engagement" on the part of the Air Force with Northrop, and its concerns, but he declined to predict if the Air Force would make sufficient changes that would allow the companies to compete after all.

Guy Hicks, an EADS North America spokesman, said: "The value of added capability offered above the minimum requirement -- including greater range, fuel offload and transport capacity -- must be included and fairly weighted in the final request for proposal."

The Northrop-EADS is offering an Airbus A330-200 wide-body derivative.

The contract could be worth $25 billion to $50 billion over time, a senior U.S. military officer said in relaunching the competition September 24.

The contract is due to be awarded by the end of June.

EADS "HEARTENED"

The potential bidders met separately on Tuesday at Wright Patterson Air Force Base in Dayton, Ohio, with government officials weighing possible changes to draft bidding rules released in late September.

"We're heartened" by the discussions under way, O'Keefe said. "We couldn't ask for a more thoughtful and forward-leaning response."

He said he understood the government was now aiming to release a final request for proposal in the middle of next month.

A Boeing spokesman, William Barksdale, said the Boeing team met Air Force officials for several hours "to voice both observations and concerns" about how tanker proposals would be judged.