First flown in 1946, the P4M lost the competition to be the Navy's next maritime patrol and anti-submarine aircraft to the smaller and less expensive Lockheed P2V Neptune. Unlike the Neptune, which had its auxiliary jet engines tacked on as an afterthought on later models, the Mercator was designed as a four engined aircraft and it's jets were integrated into the engine nacelles. Procured in small numbers, the Mercator had a distinguished career in the electronic reconnaissance or "ferret" role, in which it made close approaches to, or actual penetrations of, potentially hostile territory in order to provoke defensive systems reactions that it could measure and record. One patrol squadron (VP-21) operated the P4M-1 for a brief period, and the P4M-1Q was frequently marked in imitation of the Neptune's or other VP squadrons to disguise its ferret role. At least two Mercator's were shot down in the performance of the ferret mission. The last Mercator's were withdrawn from service in 1960.