The Mercator was an unsuccessful contender for a United States Navy requirement for a long-range maritime patrol bomber, with the Lockheed P2V Neptune chosen instead.

Work began on the Model 219 in 1944, as a replacement for the PB4Y Privateer long-range patrol bomber, optimised for long range minelaying missions, with the first flight being on 20 October 1946.

This gave greater flexibility in payload, including long torpedoes, bombs, mines, depth charges or extended-range fuel tanks.

[3] The US Navy chose the smaller, simpler, cheaper and better performing P2V Neptune for the maritime patrol requirement, but nineteen aircraft were ordered in 1947 for high-speed minelaying purposes.

[8] Data from United States Navy aircraft since 1911,[18] Naval Fighters #37: Martin P4M-1/-1Q Mercator[19]General characteristics Performance Armament Avionics