The Martin PBM Mariner—the world's largest twin-engine flying boat—was noted for its range, bomb load, ruggedness, and seaworthiness. This graceful aircraft succeeded the PBY Catalina as the U.S. Navy's principal flying-boat patrol bomber and was flown by most of the Navy's patrol squadrons at the end of World War II. While not as famous as the Catalina or its successor, the P5M Marlin, the Mariner was an important aircraft in World War II and the Korean War.
The PBM was planned by the Glenn L. Martin Company as competitor to the Consolidated PBY, but the Mariner was actually a later design. A twin-engine, gull-wing flying boat, the PBM had a deep fuselage and twin-fin tail assembly with dihedral on the horizontal tail surfaces. The wing floats were rigid on most variants. The aircraft was some 60 percent heavier than the PBY but had a greater speed and twice the payload. The planes were capable of single-engine flight; a PBM once flew for two hours with its starboard engine out.
After flying a quarter-scale, single-seat model of the design, Martin received a Navy contract for a single prototype, designated XPBM-1. Designed to carry 2,000 pounds of bombs or depth charges, the prototype first flew on 18 February 1939.
By that time the Navy had already ordered 20 production model PBM-1s plus a single XPBM-2 intended for catapult launching. That aircraft had a strengthened hull and additional fuel tanks. The Navy built a large catapult barge—designated AVC-1—to launch heavy flying boats. While XPBM-2 tests were successful, the concept was soon dropped in favor of rocket-assisted takeoffs using JATO canisters.1
The production PBM-1s entered service with patrol squadron VP-55 during September 1940—some 19 months after the first flight. The aircraft immediately began flying neutrality patrols over the western Atlantic, employed to observe and report the movement of foreign warships—including U-boats; the reporting areas were soon expanded to the east coast of Canada and down to the West Indies. Although the United States was not yet in the war, U.S. surface ships and aircraft on neutrality patrol were of significant assistance to Great Britain.
In July 1941 the capabilities of the neutrality patrol were enhanced when two PBM-1s of VP-74 as well as one PBY-5 in each of three other VP squadrons were fitted with British ASV (air-to-surface vessel) radar. These were the first operational units of the U.S. Navy to fly radar-equipped aircraft.
Although relatively complex, the Mariner's superior performance marked it as an outstanding aircraft. Hundreds more were ordered for long-range patrol, antisubmarine warfare, transport, and search-and-rescue operations.
The original PBM-1 was followed by improved combat variants. The aircraft had bomb bays within the engine nacelles, which could hold four 1,600-, 1,000-, or 500-pound bombs, or mines, or two MK 13 aerial torpedoes for a maximum weapons load of 8,000 pounds. The defensive armament consisted of up to eight .50-caliber machine guns—two each in nose, dorsal, and tail power turrets, and single guns in the waist positions. The aircraft had a crew of seven to nine.
The PBM-3D introduced a large surface-search radome immediately aft of the flight deck. The PBM-3S was an antisubmarine warfare version with increased range, the weight of additional fuel being offset by deletion of power turrets and less armor protection for the crew. The follow-on PBM-5S had electronic countermeasures equipment to detect submarine radio transmissions, magnetic anomaly detection gear, and a large searchlight, and could launch and monitor sonobuoys. A single PBM-5N was modified for all-weather operations, an oxymoron considering how Mariners operated during the war.
The PBM-3R was an unarmed cargo variant built for the Naval Air Transport Service. It had strengthened decks for 9,000 pounds of cargo or seats for 20 passengers, and large cargo loading doors. Although only a few were built, older aircraft subsequently were modified to the cargo/transport role.
The Mariner was produced in larger numbers than any other military flying boat of any nation, except for the PBY Catalina. A total of 1,366 PBMs were produced through 1947, including 54 for the Royal Air Force. Those aircraft never became operational with the British, and some were returned to the U.S. Navy. During the war Mariners were flown in the search-and-rescue role by the U.S. Coast Guard—27 PBM-3s and 41 PBM-5s were delivered through 1945.
PBMs were also flown in small numbers by the Royal Australian Air Force in the transport role, the Dutch Navy as patrol aircraft in the East Indies, the Argentine Navy, and the Uruguayan Navy.
The most numerous Mariner model was the PBM-5, with 667 aircraft being produced during the war following two XPBM-5 prototypes. This aircraft was intended as a patrol bomber and torpedo plane, in the latter role carrying four MK 13 torpedoes in its engine nacelles and two more under its wings. But no PBMs are believed to have made torpedo attacks (as did Catalinas).
After the war, Martin built 36 PBM-5A models, an amphibious aircraft with retractable landing gear. These served mainly in the Coast Guard as PBM-5G.
The PBM-5 weighed 32,840 pounds empty and had a maximum smooth-water takeoff weight of 60,000 pounds. With 4,000 pounds of bombs the aircraft had a maximum speed of 209 mph at 19,200 feet, with a combat radius of 860 miles at 135 mph.
Mariners were employed in most theaters of World War II and were again called into combat in the Korean War. They served in patrol squadrons until June 1956, the last in VP-50. A few Mariners continued in service for several more years, with some in unusual research roles, such as being a hydro-ski test aircraft. The last known PBM to fly in military livery was an Uruguayan naval aircraft, retired in February 1964.
But the soul of the Mariner continued in service when, in 1948, the final PBM-5 was modified to become the prototype for the P5M Marlin, which, in turn, became the last flying boat to see operational service in the U.S. Navy.
1. The AVC-1 was a 424-foot craft completed in 1941. She was never used operationally.
