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.

On the surface, the boats had a range of 7,050 nautical miles (13,060 km; 8,110 mi) at 9.2 knots (17.0 km/h; 10.6 mph) and 62 nmi (115 km; 71 mi) at 4 knots (7.4 km/h; 4.6 mph) submerged.

They were also armed with a QF 4.7-inch (120 mm) Mark IX deck gun.

[3] Six boats were ordered in February 1929 but, following the May 1929 general election, the new government suspended the construction of two boats in August due to economic considerations; they were subsequently cancelled.