Northrop Grumman Corp. said its shipbuilding unit on Tuesday delivered to the U.S. Navy its sixth Virginia-class submarine, the New Mexico SSN 779, four months ahead of schedule.

New Mexico’s 70-month construction period for New Mexico SSN 779 represents the shortest overall construction time of any Virginia-class submarine, the Los Angeles defense contractor said. Northrop Grumman Shipbuilding is teamed with General Dynamics Electric Boat to build the first 18 ships of the Virginia class. Current plans call for 30 such submarines in the fleet.

Separately, Northrop subsidiary Amsec LLC was awarded a Navy contract for maintenance, repair and modernization of submarines either based or docked at Pearl Harbor.

Amsec is one of six contractors awarded a five-year contract at the Pearl Harbor Naval Shipyard and Intermediate Maintenance Facility. The contract has combined value of $140.4 million to all the contractors.

Northrop shares closed up 1 cent, or less than 1 percent, to $56.84 on the New York Stock Exchange.