During the later stages of World War 2 (1939-1945), the United States Navy (USN) began an active search for a new long-ranged maritime patrol bomber to succeed an aging fleet of Consolidated PB4Y "Privateer" aircraft in same role. Two potential candidates emerged from the competing firms of the Glenn L. Martin Company (Martin) and Lockheed and these became the P4M "Mercator" and P2V "Neptune", respectively. In the end, however, the latter was selected for serial production and frontline service while the P4M was taken only into limited service to satisfy a long-ranged electronic reconnaissance role for the USN - this resulted in just nineteen aircraft being built.
The aircraft that would become the "Mercator" was known in-house as the "Model 219" and work began as soon as 1944 on the type (World War 2 ended in 1945). In its prototype form the aircraft recorded a first-flight on October 20th, 1946 and flew with 2 x Pratt & Whitney R-4360-4 "Wasp Major" air-cooled radial piston engines providing necessary power and performance to go along with the range required of maritime types (often operating over swathes of uninhabited, landless terrain).
At this stage during the "age of the jet engine", it proved somewhat common for American warplanes to feature a "combination" propulsion scheme so 2 x Allison J33 series turbojets were added to augment performance for the aircraft - either to reduce the runway take-off length required or escape from pursuing enemy interceptors. The jet systems relied on the same fuel as the prop-driven radials so only a common fuel supply was necessary.
Standard installed armament became 2 x 20mm autocannons in the nose, an additional 2 x 20mm autocannons at the tail, and 2 x 0.50 caliber Heavy Machine Guns (HMGs) in a dorsal turret. In this fashion, the aircraft would defend itself from most any position an enemy was approaching from (save the underside). The optional bomb load could be made up of conventional drop bombs, naval mines, depth charges, or torpedoes up to a 12,000lb total war load.
The aircraft was given a very slim top-down profile, its fuselage glazed at both nose and tail. The cockpit was seated aft and above the nose section as in a typical "stepped" arrangement so this elevated position could improve pilot vision over the aircraft and towards each engine. The engines were underslung at each mainplane, extending noticeably forward from the leading edges. The bomb bay buried within the fuselage was equal in length to the engines. Instead of fitting the main landing gear into the engines, the legs were positioned outboard of the nacelles and recessed into the wings (folding outwards from centerline). The nosewheel was positioned under the mass of the nose section to complete the tricycle arrangement needed for ground-running. The tail unit incorporated an elegantly-shaped single rudder fin with low-mounted, upward-canted horizontal planes. The wing mainplanes were situated directly at midships and were slim in their own right, tapering from centerline to wing tip along both the leading and trailing edges.
Two prototypes were ultimately built to the "XP4M-1" standard and these flew with R-4360-4 engines followed by the production-standard "P4M-1" models of which nineteen were built and these carried 2 x R-4360-20A series engines into service. The "P4M-1Q" designation was used to mark P4M-1 airframes converted as radar countermeasures platforms (SIGnals INTelligence = "SIGINT").
As finalized, the P4M-1 would go one to carry a complete crew of nine and, structurally, it had an overall length of 85.1 feet, a wingspan of 114 feet and a height of 26 feet. Empty weight was 48,535lb with a Maximum Take-Off Weight (MTOW) of 88,380 feet. Power was from 2 x Pratt & Whitney R-4360 "Wasp Major" air-cooled radial piston engines of 3,250 horsepower each and these were aided by 2 x Allison J33-A-23 turbojet engines of 4,600 thrust each. Performance included a maximum speed of 410 miles-per-hour, a range out to 2,850 miles, and a service ceiling up to 35,000 feet. The aircraft was fitted with the AN/APS-33 search radar.
Even as Lockheed's Neptune design went on to win the original USN requirement, the value of the Mercator was not overlooked by authorities as a mine-laying platform so a production order for the design followed in 1947 leading to service entry in 1950. As soon as 1951, the fleet was converted for the SIGINT role and operated under the aforementioned P4M-1Q designation - these flew with additional mission support equipment and more crewmembers for the revised role.
In this guise, Mercators undertook daring, often dangerous, spying missions around communist territories near the likes of the Soviet Union, China, North Korea, and Vietnam. On August 22nd, 1956, a Mercator was shot down by Chinese interceptors, killing all aboard, and another such incident followed with the Soviet Union over Mediterranean waters after the Mercator was caught near Ukraine. Similarly, North Korean interceptors met a Mercator with gunfire though the American warplane managed to survive the encounter and limp home.
Once their useful service lives were over (and pure jet aircraft took over in full), the Mercator line was given up in favor of the Douglas A-3 "Skywarrior" through its "EA-3B" guise. Unlike the Mercator, the Skywarrior was compact enough to be operated from, and be stored below, the decks of American carriers of the period. The last Mercator was retired in 1960 and none of the surviving airframes were spared.