Staff Writer (Updated: 5/11/2016):
The Mariner first appeared in 1937 as the Martin Model 162 project, giving birth to the prototype XPBM-1 as a twin engine flying boat with a high-mounted monoplane gull wing (cranked at the engine nacelles), twin tail rudders, retractable stabilizing floats and a deep hull-like fuselage. This early version featured 2 x Wright Cyclone R-2600-6 radial engines with a total output of 1,600 horsepower each and first flew in 1939. The PBM-1 followed in 1941, with some 20 models entering active US Navy service with Patrol Squadron VP-74. The first large order came for the PBM-3 and was the first in the series to feature fixed floats, a feature that would become standard in future models as well. A lengthened engine nacelle also greeted this model series and allowed for a greater external ordnance load to be carried (the bomb bays were held inside the engine nacelles).
A crew of 7 to 9 men operated the type. Armament generally consisted of a twin 12.7mm gun mounting at the bow, tail and dorsal positions with an additional 2 x 12.7mm machine guns at the beam (waist) positions firing through hatches. The bombload was a respectable 8,000 pounds (PBM-3D).
The PBM-3 would become the first widely-produced and modified model in the series with unarmed transport and passenger transport derivatives joining the base production model. Five PBM-3B's were sent to Britain via Lend-Lease for evaluation as the Marine GR.Mk I (the Royal Air Force eventually received some 32 total Mariners). The PBM-3C model was designed with standardized British and American equipment as needed along with ASV search radar (identified as fixture above the cockpit). The R-2600-22-powered PBM-3D model featured improved armor and defensive armament (also with ASV search radar). A specialized anti-submarine model appeared as the PBM-3S and featured improved range. The designation to PBM-5 appeared as the final version of the Mariner. This model sported improved defensive armament (no fewer than 8 heavy caliber machine guns) Pratt & Whitney radials and AN/APS-15 series search radar.
The Mariner primarily served in the Pacific Theater, where it was sent on reconnaissance, hunting and shadowing missions against the dwindling numbers of Japanese Navy elements. As the theater gradually collapsed on the Empire, more land-based aircraft with greater range could be counted to complete the role of the Mariner and as such the aircraft gradually fell by the wayside. Some 1,700 of the type were produced covering various models for use by American and British forces.