Shavrov Sh-2 was the first Soviet mass-produced flying boat. The very first plane was built by aircraft designer Shavrov, engineer Korvin and mechanic Funtikov. They were building it in the Korvin's flat(!) in Leningrad within six months. When in 1929 everything was finished, they lowered it piecemeal from 3d floor and transported it to the nearest airfield. They tested it in Leningrad and then did a flight from Leningrad to Moscow.
In 1932 mass production has started. Within next two years 270 planes were built, and then production was stopped. When the Winter war has started, the Red Army understood the necessity of light flying boat for communication between units and evacuation of the wounded (there are many swamps, rivers and lakes on that territory). The production has started again as well as repair of old planes. This flying boat was easy to produce and very, very cheap. The production continued in WWII, ~700 boats were built in all.
Sh-2s were continued to use long after the war by geologists, forestry officers, Arctic explorers, border guards etc. Also it used as passenger aircraft at the unpopulated territories.