Escalating the fight over the biggest defense contract in years, Boeing Co. said yesterday it would challenge the Pentagon's decision to place an aircraft order potentially worth $40 billion with rival Northrop Grumman Corp. and its European partner, Airbus.

Boeing, the world's largest aerospace company, plans to file a formal protest today to overturn what is likely to be the nation's last big new weapons contract for at least a decade.

"Our team has taken a very close look at the tanker decision and found serious flaws in the process that we believe warrant appeal," said Jim McNerney, Boeing's chief executive. "This is an extraordinary step rarely taken by our company, and one we take very seriously."

In one of the more intense competitions whose outcome has riled "buy American" proponents, the Air Force on Feb. 29 chose Northrop and Airbus over Boeing to build 179 aerial refueling tankers for the Air Force. They would replace tankers built in the late 1950s and early 1960s.

Air Force officials have not yet detailed how Northrop won the contract, but Northrop officials said it was told in a briefing Monday that its tanker was selected because it was "more advantageous" than Boeing's offer in four of the five key measures.

Those included cost, the company's past performance with other defense contracts and mission capability.

The decision has prompted a flurry of outcries, particularly from lawmakers from states with big Boeing payrolls who say that the contract would send U.S. jobs overseas and hurt the nation's defense industry.