– The U.S. Navy awarded a $5 billion contract to General Dynamics’ Electric Boat Thursday to finish designing a new class of ballistic-missile submarines so construction can start.

The Navy called the program its top priority because ballistic-missile submarines help deter nuclear war. An Electric Boat official said the award, announced Thursday, keeps the program on track.

The Connecticut-based company is the prime contractor for the Columbia-class ballistic-missile submarine program. It’s designing 12 submarines to replace the current fleet of 14 aging Ohio-class ballistic-missile submarines.

Construction on the Columbia class is expected to begin in fiscal 2021 at Electric Boat’s Rhode Island manufacturing plant and at its headquarters in Groton, Conn. Newport News Shipbuilding in Virginia is the subcontractor. The first of the 12 submarines is expected to be delivered to the Navy in fiscal 2028.

The total cost of the shipbuilding program is about $100 billion. Each submarine is being designed to stay in the fleet for 42 years.

“The Ohio-class strategic deterrent submarine is going to reach the end of its operational life here in the next decade, so it’s extremely important for the Columbia detailed design and construction to proceed so that ship is built and delivered to the Navy in time,” said Will Lennon, Electric Boat’s vice president for the Columbia-class submarine program.

Electric Boat received a $76 million contract in late 2008 to start working on a missile compartment that could be used for both the U.S. Navy’s new ballistic-missile submarine and the Royal Navy’s new ballistic-missile submarine.

In 2012, it received a five-year, $1.85 billion award to do the research and development for the Ohio-class replacement and to continue developing the common missile compartment.

Adm. John Richardson, the chief of naval operations, told Congress earlier this year that it’s “the Navy’s contribution to our nation’s strategic nuclear deterrent and our highest shipbuilding priority.”