By the late 1930s, Nazi Germany and an expansionist Japanese empire had become tangible threats to world peace. In a worst-case scenario, the isolationist United States could potentially find itself cut off from land bases on both oceans. Even before the XPBM-1 flew, Martin’s engineering staff had started work on the considerably larger four-engine Model 170. When Glenn Martin presented it to BuAer in early 1938, he touted the huge flying boat as a “sky battleship” or “flying dreadnought” able to defend itself and deliver a 10,000-pound bombload (2½ times that of the Boeing YB-17) over long distances from sea bases. The Navy was sufficiently impressed to order one prototype in August 1938 as the XPB2M-1. Due to other factory commitments, Martin was unable to begin building the aircraft until mid-1940, and it was not ready for taxiing tests until early November 1941.

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In line with the company practice of choosing names that started with “M,” the big flying boat was dubbed Mars. Sharing the general hull design and twin-fin arrangement of the PBM, the XPB2M-1 was powered by four Wright R-3350-8 engines rated at 2,200 hp each. It boasted a wingspan of 200 feet, a length of 117 feet 3 inches and a loaded weight of 144,000 pounds. Planned armament consisted of five turrets, each mounting one .30-caliber machine gun, and a bomb bay under each wing holding up to five 1,000-pound bombs. At that time it was the largest flying boat and the third largest aircraft of any type in the world (the Tupolev ANT-20bis of 1938 and the Douglas XB-19 of 1941 were fractionally larger).

The program experienced a critical setback during taxi tests when the No. 3 engine threw a propeller blade and caught fire. By the time the XPB2M-1 was towed back to shore and the fire extinguished, the aircraft had suffered serious damage to its starboard wing and No. 3 nacelle, as well as structural damage where the propeller blade had penetrated the fuselage. It took Martin more than six months to complete repairs, and the XPB2M-1 did not make its first flight until July 2, 1942. In the interval, the U.S. had declared war on Japan and Germany and had already fought two important naval battles in the Pacific. This experience caused naval planners to completely discard the notion of the sky battleship in favor of smaller combat aircraft operating from carriers, together with greater numbers of twin-engine flying boats (i.e., PBYs and PBMs) employed for maritime patrols, effectively leaving the big Mars without a combat role.

Around the time the XPB2M-1 first flew, the U.S. was confronted with a serious threat to its Atlantic shipping lanes from marauding Ger­man U-boats. Fleets of large flying boat transports—“sky freighters” that would be immune from torpedo attack—were seen as a potential means of moving large quantities of war materiel to Great Britain and other battlefronts. Thus it came as no surprise in early 1943 when BuAer authorized Martin to remove all armament and bombing gear from the Mars prototype and modify it as a transport under the new designation XPB2M-1R. To boost production, industrialist Henry J. Kaiser offered to license-build the Mars by the hundreds in his West Coast shipyards. Glenn Martin expressed no interest in sharing his production rights with another manufacturer, however, and Kaiser moved on to better prospects.

Factory testing of the transport conversion was completed in mid-1943, and the XPB2M-1R was delivered to the Navy for operational evaluation and crew training later the same year. By that time, however, the tide of the war had turned in favor of the Allies on both ocean fronts, and there was no longer a need for large numbers of sky freighters. (This turn of events also resulted in the cancellation of other big flying boat projects, including Howard Hughes’ gargantuan 400,000-pound H-4 Hercules and the proposed 250,000-pound Martin Model 193, a six-engine 25 percent scale-up of the Mars.)

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