The U.S. Navy has extended its contract with Boeing Co. for P-8A Poseidon airplanes, the company said Thursday.

It ordered 13 more of the planes, which are based on a 737 airframe and are used for anti-submarine, anti-surface and surveillance missions, for $1.98 billion.

The deal is the extension of an existing contract that is ultimately planned to include 100 Poseidons. The Navy has now ordered 37 and Boeing has delivered 10, with the first planes due to be operational by the end of the year.

The P-8 is assembled at Boeing's 737 production facility near Seattle.

Boeing's Defense, Space and Security unit is headquartered in Hazelwood.