With the Martin M-130 flying boat operating on Pan American's prestigious trans-Pacific route between San Francisco and Hong Kong, the airline in 1937 sought out newer and larger equipment to expand the Pacific network. Martin was the incumbent, having built three M-130s for Pan American that were christened "China Clipper", "Philippine Clipper", and "Hawaii Clipper." The M-130 served as the basis for enlarged derivative designated the M-156 which had essentially an M-130 fuselage mated to an enlarged wing with four more powerful radial engines and a twin-fin tailplane mounted on a pylon on the aft fuselage.
In a surprise to many, Pan American rejected the M-156 and instead with the M-156's competitor, the Boeing 314 flying boat. The Boeing 314 was a bigger, faster and more spacious aircraft that easily won the airline's chairman, Juan Trippe, over with comforts that were possible due to its bigger size.
Martin, however, had already been in negotiations with the Soviet Union for the M-156 not just for purchase, but also for license production. The M-156 design was far more advanced in design and capability than what either was in production in the Soviet Union or up for sale by the nations of Western Europe. The contract signed with the Soviet government was for the transfer of production drawings and equipment to allow the Soviets to build their own M-156s for Aeroflot passenger services in the Russian Far East. In December 1937, the M-156 prototype was transferred to the Soviet Union along with a series of production drawings that had to be converted from imperial units to the metric system (which would lead to problems for the Soviets). Rather unusually, though, it was transported by ship to Leningrad rather than being flown across the Atlantic.
Plans were in place, however, to use the M-156 (dubbed "Soviet Clipper" by Martin) as the basis of a long range bomber operated by the naval aviation. A full-scale fuselage mockup was built that incorporated bomb bays and defensive gun turrets. Stalinist purges of the aircraft industry, though, impeded progress on the bomber version of the M-156 but the prototype aircraft by 1940 was already flying passenger services in the Russian Far East for Aeroflot. As the Second World War dragged on, spare parts problems, fuel shortages and accidents cut into the Soviet Clipper's flying hours and by 1944 the aircraft was effectively grounded and is believed to have been scrapped sometime around 1946.
Source: Pan American's Clippers- The Golden Age of Flying Boats by James Troutman. Boston Mills Press, 2007, p49. Supplemental material from www.airwar.ru/