Two boats were replacements for submarines of that class that were secretly transferred to the Nationalists during the Spanish Civil War in 1937.

[1] For surface running, the boats were powered by two 1,500-brake-horsepower (1,119 kW) diesel engines, each driving one propeller shaft.

On the surface, the Brin class had a range of 9,000 nautical miles (17,000 km; 10,000 mi) at 8 knots (15 km/h; 9.2 mph), submerged, they had a range of 90 nmi (170 km; 100 mi) at 4 knots (7.4 km/h; 4.6 mph).

[2] The boats were armed with eight internal 53.3 cm (21.0 in) torpedo tubes, four each in the bow and stern.

The light anti-aircraft armament consisted of one or two pairs of 13.2 mm (0.52 in) machine guns.