The U.S. Navy has awarded Boeing Co. BA, -3.81% a $2.5 billion modification to a previously awarded, fixed-price contract for manufacturing and delivering 16 P-8A "Poseidon" aircraft for the Navy and four for Australia, according to a Department of Defense web site. The Poseidon's primary function includes anti-submarine warfare, and intelligence, surveillance and reconnaissance. Shares of Boeing were flat in late trading Thursday after ending the regular session up 1.2%.