The torpedo entered service with the Allies in March 1943; the United States Navy (USN) used it until 1948.

Western Electric was to develop a lightweight, shock resistant, 48 volt lead-acid battery capable of providing 110 amps for 15 minutes.

General Electric was to design and fabricate propulsion and steering motors and to investigate an active acoustic homing system.

The guidance system used four hydrophones on the torpedo's midsection, connected to a vacuum tube-based sound processing array.

A Bell proportional navigation and HUSL non-proportional steering system were demonstrated by July 1942.

Testing of the pre-production prototypes continued into December 1942, and the USN received the first production models in March 1943.

Upon water entry, FIDO performed a circular search at a predetermined depth controlled by a bellows and pendulum system.

The circular search resumed above depths of 40 feet (12 m) to prevent attacks on surface ships.