US aerospace giant Boeing said it would use a modified 767 plane in its bid for a massive US Air Force refueling tanker project.

The formal announcement that Boeing would offer the KC-767 was widely expected, and opens up a fierce bidding competition between Boeing and Northrop Grumman, which is bidding with its European partner EADS, parent of Airbus.

"The Boeing KC-767 Advanced Tanker is made for this mission," said Ron Marcotte, vice president and general manager of Boeing Global Mobility Systems.

Marcotte said the 767 was "the ideal fit" for the mandate of a refueling aircraft, "often in tight, hostile locations."

He said the Boeing plane was "highly energy efficient, agile and with exceptional takeoff performance."

The Pentagon said last week it was committed to an open bidding for the checkered "KC-X" project, which aims to replace the Air Force's aging mid-air tankers under a deal that could be worth up to 200 billion dollars.

A year ago, the contract was re-opened after a procurement scandal sank an initial Air Force deal to buy Boeing tankers without a competitive bidding process.

The Pentagon has dropped any link between the bidding and a legal tussle over aircraft subsidies being waged by the United States and European Union at the World Trade Organization.

The draft "WTO clause" was seen as penalizing EADS, whose commercial aircraft unit Airbus is the target of the US government case at the Geneva-based referee of the global trading system.

The initial Air Force deal covers 179 tanker planes, worth around 30 to 40 billion dollars. The total contract, due to be signed by the end of the year, is estimated at about 200 billion dollars.

The US Air Force wants to replace its Boeing KC-135 Stratotankers, which date from the 1960s.

Northrop had earlier this year threatened to back out of the project, complaining the Pentagon's initial requirements for the new tanker unfairly favored Boeing.

Without a US partner, EADS would then have been forced to withdraw also. Such a move would have been an embarrassment for the Pentagon after the earlier tanker fiasco.

A congressional committee in 2004 voted to kill a hotly contested deal that would have allowed Boeing to lease 767 jetliners to the Air Force as refueling tankers.

Shortly afterward, Darleen Druyun, a former Air Force official who steered the tanker contract to Boeing and then later went to work for the Chicago-based company, was sentenced to nine months in prison for her role in the scandal.

In 2005, former Boeing chief financial officer Michael Sears was sentenced to four months in jail and fined 250,000 dollars for his role.