WASHINGTON — To pursue a massive contract to continue developing and sustaining the United States’ primary territorial missile shield, incumbent Boeing says it will partner with Northrop Grumman to take on challenger Lockheed Martin and its newly announced partner, Alaska Aerospace Corp.

The U.S. Missile Defense Agency (MDA) is preparing to hold a new competition for a single prime contractor to manage all operations, maintenance and modernization for the Ground-based Midcourse Defense (GMD) system. The contract is expected to be worth around $600 million a year. The MDA issued a draft request for proposals for the effort in May, and the agency intends to select a prime contractor in early 2011.

Boeing Defense, Space & Security of St. Louis has been the GMD prime contractor since the program’s inception in 1998, responsible for the development, deployment, integration and testing of the system. Boeing will pursue the new contract as the prime contractor with Northrop Grumman as a teammate, a June 14 Boeing press release said. Los Angeles-based Northrop Grumman developed the GMD command, control and communications system and also is responsible for sustaining the U.S. fleet of ICBMs.

Greg Hyslop, vice president of Boeing Missile Defense Systems, said Boeing and Northrop Grumman’s combined experience — the two companies already perform much of the GMD development and sustainment work — makes the Boeing-Northrop team the obvious frontrunner in the competition.

“With the Boeing-Northrop team, there’s no learning curve to come up to keep GMD at high levels of availability and provide any development for the future of it,” Hyslop said in a June 17 interview.

Lockheed Martin, which was bested by Boeing for the original GMD contract, announced June 17 it would team with state-owned Alaska Aerospace Corp., which operates the Kodiak Launch Complex.

Raytheon Co. of Waltham, Mass., previously announced its intention to bid for the contract, but spokesman John Patterson would not reveal any teaming arrangements.