The Royal Canadian Navy (RCN) was assigned responsibility for anti-submarine warfare (ASW) and controlling sea space in the western North Atlantic.

Design work for a new class of destroyer escorts began in June 1949[3] with the original completion date slated for 1955.

[4] The St Laurent class were built to an operational requirement much like that which produced the British Type 12, and were powered by the same machinery plant.

They were built to counter nuclear, biological and chemical attack conditions, which led to a design with a rounded hull, a continuous main deck, and the addition of a pre-wetting system to wash away contaminants.

The remaining 7 vessels were built as the follow-on Restigouche class to incorporate advancements in naval warship design in the preceding years.

[9] The St. Laurent class was fitted with twin 3-inch (76 mm)/L50 calibre guns in two mounts for engaging both surface and air targets.

As with the British Type 12 design, the provision for long-range homing torpedoes (in this case BIDDER [Mk 20E] or the US Mark 35 were included.

[note 2] The vessels of the St. Laurent class had two Babcock & Wilcox water tube boilers installed[9] providing 600 psi (4,100 kPa), 42 (kgf/cm²) at 850 °F (454.4 °C).

[7] The advent of nuclear-powered attack submarines in the late 1950s prompted RCN leaders to assess the new threat they posed.

During a 25 February 1959 meeting of the Naval Board, it was decided that the Navy would counter the new threat by outfitting destroyers for helicopter operation.

[13] Trials held in October 1956[12] using a Sikorsky HO4S-3 were successful,[14] and a larger temporary helicopter landing platform was installed in the new destroyer escort HMCS Ottawa in August 1957.

[15] Operational trials were conducted using an RCAF Sikorsky S-58, a substantially larger and heavier aircraft than the HO4S, and the success of these tests led to approval of the concept.

[14][16] To achieve the goal, the RCN needed a helicopter capable of all-weather day-and-night operations with a heavy weapons load—capabilities the HO4S lacked—and a means to handle and secure the aircraft on the landing platform in rough seas.

[9] Initial studies identified two helicopters that met the upcoming requirements- the Sikorsky S-61 (HSS-2) Sea King and the Kaman K-20 (HU2K).

[19] Note that the pennant numbers were originally prefixed with the classification symbol DDE but were changed to DDH in the early 1960s.

[9] Assiniboine joined the RCN at Halifax, Nova Scotia following her commissioning and spent the next two years on the east coast.

In 1959, the ship transferred to the west coast and served there until being taken out of service in 1962 for conversion to a helicopter-carrying destroyer, the first of the class to go through the process.