LONDON—The 100th Boeing P-8 Poseidon maritime patrol aircraft has entered final assembly. The aircraft, which is receiving its wings and tail at Boeing’s plant in Renton, will be delivered to the U.S. Navy this autumn. The milestone comes as Boeing works with the U.S. government to further expand the customer base for the maritime patroller, which is slowly edging the long-serving Lockheed P-3 Orion out of front-line use. Boeing currently is producing aircraft for Australia and ...