The submarine fleet available to the Italian Royal Navy (the Regia Marina) in World War II was the product of a long term build-up during the interwar years, despite the prevailing economic and political pressures of the period.
To this end the Regia Marina made plans for a fleet of vessels in three Types: At the same time the major powers were negotiating an arms limitation treaty at the 1922 Washington Naval Conference.
In the inter-war years : between 1925 and 1929 Italy built a series of ocean-going submarines in a number of small classes, in order to find the most suitable designs for expansion.
Just prior to war, in 1939, the Italians also commissioned the Cagni class, designed specifically as a commerce raider with a long range and armed with 14 Torpedo Tubes of 17.7in calibre (these being more suitable against merchant ships).
She also developed a manned torpedo, the SLC, an update of an Italian First World War design, for use by the Navy's special forces.
This was the Flutto class of submarines, an enlarged 600 series design for medium-range use, with mass production under wartime conditions in mind.
Bernardis favoured a single hull design, for better submerged characteristics, but adding side blisters for stability on the surface.
One feature that caused problems was the large conning tower, making the boat more visible on the surface, and slowing the dive time.
Results were modest, with only 21 merchantmen and 13 enemy warships sunk (for a total of around 100,000 tons); one reason for such a disappointing score was the lack of targets (with most of them being harder-to-hit warships, and the merchant ships being under heavy escort), and another was the outdated doctrine employed at the beginning of the war (with static patrols, and attacks being executed by firing only one or two torpedoes), although this aspect was being corrected by 1942 (as proven during Operation Pedestal, when a more aggressive and dynamic conduct met with considerable success).
[7] In 1943 at Italy's surrender the Regia Marina had 34 boats operational, having lost 92 vessels in action (over two-thirds of their number).