Thus, in an attempt to obtain up-to-date design information, Boris Malinin was attached to a Soviet naval mission which embarked on a tour of shipyards in Western Europe in 1925.

[1][2] Although Germany's socialist government persuaded their reluctant navy to share some older designs from the First World War,[3][4] sources agree that the most useful information was obtained in Italy.

Some authorities state that these were plans for the brand-new ocean-going Balilla-class submarine, impressively large boats (1,450 tons surface displacement, 86.5 metres long) which had been laid in down in early 1925.

[2][5] Others assert that they were instead the plans of a much older and smaller submarine of the same name, laid down as far back as 1913 (717 tons surface displacement, 65 meters long), which had been obtained in a second-hand bookshop in Rome, and were of little real value.

Probably related to this is a claim that Malinin obtained plans for the Pisani class built by one of CRDA's precursors CNT, construction work on which was just beginning during the delegation's visit to Italy.

[14][12] At 76 metres (249 ft) in length, and with a surfaced displacement of around 1,000 tons, they were significantly larger than any earlier submarines in the Soviet fleet (or any built in Imperial Russia), though not as large as the new Italian Balilla class.

[19] An order of six boats was divided equally between the Baltic and the Black Sea Fleets of the Soviet Navy: three vessels were constructed by the Ordzhonikidze Shipyard in Leningrad, and the other three at the Marti Yard at Nikolayev.

[16][21] Bazilevsky later insisted that the only real problem was an asymmetrical weight distribution, which could be corrected by a small quantity of ballast, but which had not been detected at the start of the trial due to a miscalibrated inclinometer.

[23] In May 1933 the three Baltic boats were transferred to the Northern Fleet, in the Arctic Ocean, via the White Sea–Baltic Canal, and are said to have showed high seaworthiness in polar circumstances,[citation needed] although D-1 was lost with her entire crew in a diving accident on November 13, 1940, in Motovsky Gulf.