Velella was one of a pair of submarines ordered by the Portuguese government, but taken over and completed for the Regia Marina (Royal Italian Navy) during the 1930s.

[1] For surface running, the boats were powered by two 750-brake-horsepower (559 kW) diesel engines, each driving one propeller shaft.

On the surface, the Argo class had a range of 10,176 nautical miles (18,846 km; 11,710 mi) at 8 knots (15 km/h; 9.2 mph); submerged, they had a range of 100 nmi (190 km; 120 mi) at 3 knots (5.6 km/h; 3.5 mph).

They were also armed with a single 100-millimeter (3.9 in) deck gun, forward of the conning tower, for combat on the surface.

[1] Velella was built by Cantieri Riuniti dell'Adriatico in its Monfalcone shipyard.