The PBM (Patrol Bomber, Martin) Mariner is one of the least known patrol aircraft of World War Two, yet it was also one of the most successful. While the Consolidated PBY Catalina (see NASM collection) proved to be one of the most versatile maritime patrol aircraft of the conflict, by the end of the war, the larger, faster, and more capable PBM supplanted it in many of its roles. Even though the Mariner entered service before the war, sufficient numbers were only available towards the end of the conflict to begin replacement of the numerous, but obsolescent Catalinas.
The Glenn L. Martin Aircraft Company competed with the Consolidated Aircraft Company throughout the early 1930s for new Navy patrol aircraft contracts. However, in 1933 Martin abandoned military aircraft for the promise of the much more lucrative Pan American Airways "China Clipper" contracts that resulted in the remarkable Model 130. However, demand for increased capacity soon led to Pan Am's selection of the most magnificent of the great flying boats, the Boeing 314, which forced Martin's flying-boat design team to return grudgingly to designing long-range maritime patrol aircraft. In 1936, they unveiled a design for the fast four-engined Model 160 patrol bomber.
The company lacked sufficient funds to undertake a large-scale development program without outside support and decided instead to construct a 1/4-scale proof-of-concept flying prototype of the Model 160. Consolidated, fearful of losing its monopoly on Navy patrol aircraft, exerted pressure on the Navy to hold off on issuing a production contract for Martin's design, until the company successfully demonstrated a full-scale prototype, designated as the XPBM-1. In the meantime, Martin's management realized that four-engined patrol bombers were not likely to be cost-effective in the Navy's eyes and revised the Model 160 into the two-engined Model 162. Martin, in close competition with Consolidated and other firms, could not afford the time to construct a full-scale prototype and decided to risk the Navy's disapproval by altering the plans for the 1/4 scale Model 160 and building it as a 3/8 scale Model 162. This new prototype, known as the 162A "Tadpole Clipper," began testing in November 1937 and its performance quickly earned the company the production contract it desired in spite of its circumvention of the Navy's full-scale prototype requirement. In the meantime, the rising tensions in Europe increased the urgency for maritime patrol aircraft, and the Navy decided to fund PBM development without a full-scale prototype.
In 1938, design and construction of the XPBM-1 (Martin Model 162) began in earnest. It incorporated several improvements over the PBY, including a deep hull mounting a gull wing, which kept the vulnerable engines out of the salt spray without the use of the Catalina's parasol wing and its external bracing. An internal bomb bay allowed higher speeds when the aircraft was loaded with ordnance. To reduce drag, the wing floats could retract into wing bays. The improved streamlining, combined with two 1,600 hp Wright R-2600-6 engines, gave the much larger XPBM-1 a speed advantage over the PBY. On production PBM-1s, an upwardly-canted horizontal stabilizer replaced the earlier level version, which resulted in an inward tilt of the twin vertical stabilizers and gave the aircraft a very distinctive appearance.
On February 18, 1939, the XPBM-1 made its first flight, but delivery of the first 21 production PBM-1s did not occur until September 1940. The PBM went into service as a patrol aircraft that could not only locate enemy shipping, but also attack it with a 1,814 kg (4,000 lb) bomb load (or torpedoes in later versions). The aircraft, with its crew of nine, was also very capable of defending itself against enemy aircraft with .50 caliber machine guns individually mounted in the nose and dorsal turrets, as well as in the waist gimbals and tail position. A hand-aimed .50 caliber machine gun mounted in the tail, and a .30 caliber machine mounted in the floor of the tunnel that linked the tail gunner position with the main cabin, rounded out the potent defensive armament.
In October 1940, the first PBM-1s went into service with Navy patrol squadrons VP-55 and -56. The Mariners were soon patrolling the Atlantic in search of the German U-boats that threatened to disrupt the supply of war material to Great Britain. As American involvement in World War Two became imminent, the Navy ordered 379 improved PBM-3 models that incorporated upgraded 1,700 hp Wright R-2600-12 engines, power-operated turrets, and fixed wing floats that provided greater strength for open-ocean operations.
The PBMs proved their value in combat during the Battle of the Atlantic, during which they operated out of bases located in the western Atlantic, ranging from Bermuda to Brazil. In July and August 1943, Germany's U-boat force waged an intense campaign in the West Indies and Caribbean against allied shipping. Mariners sunk five of the ten submarines involved in the operation. However, by this time, the U-boats had dramatically increased their anti-aircraft armament, including the dreaded Flakvierling 38 quad 20mm system. U-boats shot down three Mariners during the battle and heavily damaged eight others. Of the twenty-nine submarines sunk by American patrol aircraft during World War, ten were by PBMs.
The PBM-3 spawned a series of variants for specialized missions, including the PBM-3C, which mounted a powerful APS 15 radar and dual .50 caliber machine guns in its three turrets, in addition to the original single .50 caliber guns mounted in the waist windows on either side. The PBM-3S was a stripped down version for long-endurance anti-submarine patrols. The PBM-3R transport version had its armor and armament removed. These aircraft supplied distant submarine bases that lacked conventional airstrips. Later they flew medical evacuation missions - a role for which the aircraft's spacious hull was well suited. PBM-3Cs went to Great Britain in 1943 under Lend-Lease, but the British preferred the Short Sunderland for their needs and sent the Mariners back. However, the British did convey the "Mariner" sobriquet to the aircraft. The U.S. Navy soon followed.
In 1944, the final version of the Mariner, the PBM-5, entered service. This model included upgraded radios, turrets, and radar, in addition to new R-2800 engines that were more reliable than the troublesome R-2600s installed on the PBM-3. One of the most innovative features of this version was the provision for Jet Assisted Takeoff (JATO) bottles. These were ideal for making short takeoffs in the heavy Pacific sea conditions frequently encountered on "Dumbo" rescue missions flown for B-29 crews that had to bail out or ditch.
As the submarine threat diminished towards the end of the war, PBMs operated in other combat roles, such as the "Nightmare" raids conducted against Japanese shipping under cover of darkness. Mariners also bombed shore installations on a number of Pacific islands, in addition to targets on mainland Japan. PBMs also claimed a number of Japanese fighters shot down in self-defense.
The Mariner continued in post-war use, and supported the early atomic bomb tests in the Pacific and mapped Antarctica under the command of Admiral Byrd. The PBM's lack of landing gear became a liability, because it could not operate from the well-equipped air bases that had become common in the areas served by the type. As a solution, the Navy ordered Martin to construct 36 PBM-5A amphibious models with retractable tricycle landing gear. A further four switched to this configuration from standard PBM-5s. The new models entered service in 1948, and 1949. Eventually, most became transports and had their turrets faired over. The Mariners swan song occurred during the Korean War, when they performed essential nighttime patrols along the Korean coastline with high-powered searchlights. In 1952, the superb Martin P5M-1 Marlin began to roll off the assembly line, and quickly rendered the Mariner obsolete. The last PBM in operational service retired from the Coast Guard in 1958.
Only one of the 1,366 Mariners constructed survives intact to the present-day. The Navy accepted this PBM-5A (Navy serial number 122071) in 1948 and assigned it to Patrol Squadron 33 (VP-33), and then to an aviation indoctrination program for Naval Academy Midshipman. In 1953, it was retired with 1,326 flight hours. It then sold as government surplus to several civilian owners who attempted to turn it into everything from a fire-fighting water tanker to a fresh fish hauler. However, the Mariner was not economical to operate as a civil aircraft and the Smithsonian Institution accepted the donation of 122071 in 1973. This aircraft, which was one of the last PBMs produced, provides an interesting contrast to the prototype Model 162A "Tadpole Clipper," also in the museum's collection.
The aircraft is on loan and displayed at the Pima Aerospace Museum in Tucson, AZ.
