The Rainbow-class submarines were designed as improved versions of the Parthian class and were intended for long-range operations in the Far East.

[2] For surface running, the boats were powered by two 2,200-brake-horsepower (1,641 kW) diesel engines, each driving one propeller shaft.

In October 1939 Regulus conducted intelligence-gathering operations in the mouth of the Bungo Strait, off the coast of Japan.

Frederick Basil Currie, RN) left Alexandria to patrol in the southern Adriatic on 23 November 1940.

She was lost with her entire crew on 6 December 1940 whilst on patrol off Taranto, Italy.