UPDATE 2-Boeing may offer US Air Force 2 tankers

* Boeing considers offering versions of both 767 and 777

* Northrop Grumman defers comment on its plans

* Air Force publishes detailed draft bidding rules (Adds analyst comment, quotes, background)

WASHINGTON, Sept 25 - Boeing Co said it might offer versions of both its 767 and 777 aircraft against the rival team of Northrop Grumman Corp and EADS in a renewed $35 billion contest to build a refueling fleet for the U.S. Air Force.

Boeing said on Friday it was deciding whether to stick with its modified 767 tanker, which lost a previous, canceled competition, or offer a larger 777-based tanker -- or both.

The Air Force and the Defense Department jointly spelled out Thursday how the winner of the lucrative deal would be picked. ID:nN24469923

Boeing said it is reviewing the detailed draft bidding rules published on Friday by the Air Force.

"We want to understand how requirements will be defined and prioritized and how the proposals will be evaluated," said William Barksdale, a Boeing spokesman, in a statement. "That information will help us decide which plane to offer or whether to offer both planes."

Northrop Grumman, which is offering a tanker based on EADS's Airbus A330 aircraft, is deferring public comment until it has completed a review of the draft request for proposal, said Randy Belote, a company spokesman.

The companies have 60 days to comment on the Air Force's draft rules before final bidding specifications are released.

The Air Force plans to award a contract by June, for an initial batch of 179 tankers, which refuel other planes in mid-air. The new aircraft would replace aging KC-135 tankers that have an average age of 50 years.

The plan calls for delivery of planes to begin in 2015, with the first ones operational in 2017. Two successive competitions would take place in the decades to come to complete an Air Force fleet renewal expected to cost more than $100 billion for up to 600 new tankers.

SPLIT BUY POSSIBLE?

In Congress, the competition pits Boeing backers from Washington state and Kansas, where the company would do much of its tanker work, against lawmakers from Alabama, where the Northrop-EADS team would do final assembly of its planes.

Boeing, the Pentagon's No. 2 supplier by sales, ahead of third-ranked Northrop, is eager to prevent archrival Airbus and EADS from obtaining a strategically important foothold on U.S. soil. With a plant in Mobile, Alabama, EADS could take advantage of foreign exchange and labor rates on either side of the Atlantic.

Rob Stallard, a defense industry analyst at Macquarie (USA) Equities Research, predicted the Air Force would end up buying tankers from both Boeing and Northrop because of political forces. The Defense Department has said it opposes a split buy.

"The Boeing and NOC/EADS camps on this are so entrenched that the administration would create a lot of disgruntled congressmen if they were to pick either team over the other," Stallard said in an email to Reuters.

"At a time when the government is trying to get a lot of important legislation through the Congress, they do not need this being held up by the tanker," Stallard said.

A year ago, the Pentagon scrapped a tanker deal awarded to Northrop and EADS after the Government Accountability Office, the audit arm of Congress, upheld a protest by Boeing. The GAO found the Air Force failed to follow its own rules in evaluating the bids.

The first U.S. attempt to award a tanker contract collapsed in 2004 amid an ethics scandal that sent a former top Air Force weapons buyer and Boeing's former chief financial officer to prison.