Le Tonnant was part of a fairly homogeneous series of 31 deep-sea patrol submarines also called "1,500-tonners" because of their displacement.

Propelled on the surface by two diesel engines producing a combined 6,000 horsepower (4,474 kW), they had a maximum speed of 18.6 knots (34.4 km/h; 21.4 mph).

When submerged, their two electric motors produced a combined 2,250 horsepower (1,678 kW) and allowed them to reach 10 knots (19 km/h; 12 mph).

Also called "deep-cruising submarines", their range on the surface was 10,000 nautical miles (19,000 km; 12,000 mi) at 10 knots (19 km/h; 12 mph).

[3] Prior to their departure, an incident occurred on board Le Tonnant while she was conducting diesel engine tests at La Seyne-sur-Mer in which two members of her crew plotted to seize control of her and deliver her to the Spanish Republican Navy for service in the Spanish Civil War.

[3] Italy declared war on France on 10 June 1940 and joined the invasion, and that day Le Tonnant departed Bizerte to patrol off Cap Bon on the coast of Tunisia.

[3] Le Tonnant was rearmed on 8 April 1941 and subsequently deployed to Casablanca in July 1941 and to Dakar in November 1941.

[3][7] As of 1 November 1942, she was part of the 4th Submarine Division at Casablanca with Le Conquérant, soon joined by their sister ship Sidi Ferruch.

At 06:15, the U.S. Navy aircraft carrier USS Ranger (CV-4) and escort aircraft carrier USS Suwannee (CVE-27) launched an airstrike against Casablanca, and SBD Dauntless dive bombers and TBF Avenger torpedo bombers armed with bombs began attacking targets in the harbor at 07:10.

[3] At 08:50 on 10 November, she detected a United States Navy task force north of Casablanca and launched all four of her torpedoes at Ranger, which avoided them.

[3] Unable to return to Casablanca, Le Tonnant proceeded to Cádiz in neutral Spain, arriving there on 14 November 1942.