WASHINGTON — The U.S. Missile Defense Agency (MDA) intends to issue a final request for proposals around June 30 for the lead industrial role on the nation’s primary strategic missile shield, according to a May 14 posting on the Federal Business Opportunities website.

The MDA will be seeking a single prime contractor to be responsible for the ongoing development and sustainment of the Ground-based Midcourse Defense (GMD) system for as long as 10 years, the posting said. The agency previously stated the contract is expected to be issued in early 2011 and be worth around $600 million a year.

Boeing Defense, Space & Security of St. Louis has been the GMD prime contractor since 1998, responsible for system development and operations as well as sustainment of its interceptor fields in Alaska and California. Lockheed Martin, Northrop Grumman and Raytheon all have said they intend to challenge Boeing for the new contract.

The statement of work released May 14 said the new GMD prime contractor will be responsible for upgrading the system to allow for flight testing of two interceptors simultaneously. With GMD interceptors costing some $70 million apiece, the MDA has previously said so-called salvo testing was cost-prohibitive. The GMD system has a flight test record of eight intercepts in 14 attempts since 1999.