The Boeing Co. (BA) doesn’t believe it can win the controversial competition for a $35 billion U.S Air Force tanker contract without a six-month extension, and now has threatened to pull out of the competition altogether.

The Chicago-based Boeing said it wants the extension to incorporate what the aerospace giant contends are essentially new requirements calling for a bigger tanker with a larger fuel-carrying capacity. The Pentagon has told the bidders it will grant two months for the bidders to assemble and submit their new proposals, a Boeing official told the Atlanta Business Chronicle.

“If we’re unable to secure sufficient time to prepare a competitive proposal, there’s little option for Boeing other than not to submit a bid,” Boeing spokesman Dan Beck told the Georgia-based business journal.

This latest development in the politically charged competition for the aerial refueling tankers could push out the delivery date for the much-needed aircraft by another year. But it is also expected to give Boeing more time to build up political support for its new bid, MarketWatch.com reported.

Back in February – in what some industry experts viewed as a surprise decision – Boeing lost the initial contract to a partnership of the Los Angeles-based Northrop Grumman Corp. (NOC) and Airbus SAS, Boeing’s arch-rival in the commercial jetliner business and a subsidiary of the European Aeronautic Defense and Space Co. NV (EADS).

Boeing had offered the Air Force a militarized version of its 767-200ER jetliner platform, but the Air Force opted for Northrop Grumman’s larger Airbus A330 model instead. The bidding process was subsequently reopened when Boeing won a protest with a government watchdog agency after government auditors found “significant errors” in the Air Force’s decision, Forbes.com reported.

Since that time, the Air Force has repeated it prefers an aircraft that can hold more fuel and cargo such as the A330 – essentially leaving the contract open for Northrop to grab as Boeing is forced to scramble to find a replacement aircraft it can use as the basis for an aerial tanker. Boeing’s 777 model is likely too costly because of the high demand from commercial airlines, while its 767-400 would require improvements in both flight range and endurance, said Teal Group analyst Richard L.