UPDATE 1-Tanker announcement expected Friday: US official

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WASHINGTON Feb 27 (Reuters) - The U.S. Air Force's top weapons buyer said on Wednesday her best bet was that the winner of a potential $40 billion refueling aircraft contract would be announced after U.S. markets close on Friday.

"That's my best bet for now," Sue Payton, assistant secretary for acquisition, told Reuters after a hearing of the House of Representatives Armed Services Committee on the Air Force 2009 budget request.

Boeing Co (BA.N) is vying for the contract against a team made up of Northrop Grumman Corp (NOC.N) and Europe's EADS (EAD.PA).

"It's absolutely not going to happen today," Payton said. "We're really making sure all the 'i's' are dotted and 't's' are crossed."

Air Force Secretary Michael Wynne earlier told Reuters he hoped an award would be announced "any day now," once the Pentagon's top weapons buyer, John Young, has signed off on he plan to buy 179 tankers, used to extend warplanes' operating range.

"You don't have a decision until all the paperwork is done and complete," Wynne said during a break in testimony to the House panel.

The Air Force presented its acquisition plan on Monday to a top-level panel chaired by Young, the deputy undersecretary for acquisitions. (Reporting by Jim Wolf, editing by Mark Porter)