Just before America's official involvement in World War II, Fido was born. It took a while for Fido to be ready to serve, though. Only 4,000 were fielded – down from a planned 10,000 — largely because Fido was so effective.
For Fido, though, the mission was a one-way trip.
Now, you dog lovers out there, don't go flying off the handle. Fido wasn't some poor canine conscripted for use in war to be blown to bits while killing the enemy. No, this "Fido" — as the sailors who used it against enemy subs took to calling it — was purely machine. A torpedo, to be exact.
Okay, technically Fido's designation was as the Mk 24 Mine, but this torpedo was unique in that it could sniff out enemy submarines.
According to UBoat.net, Fido's "nose" consisted of four hydrophones placed at equidistant points around the body of the Mk 13 aerial torpedo. These gave the torpedo steering directions as they detected the skulking submarine and guided the torpedo to a direct impact on the hull. That's when a 100-pound high-explosive warhead would do its job. The result should be a sunken enemy submarine.
Fido could go at a speed of 12 knots and its batteries would last for 15 minutes. It could be dropped from up to 300 feet high by planes going as fast as 120 knots. Submarines could increase their speed to try to outrun it, but their batteries would run out very quickly, forcing them to the surface, where they'd be sitting ducks to American guns. If they didn't go fast, the torpedo would catch them.
Fido was used on anything from a TBF Avenger to the PBY Catalina. It took a little less than a year and a half for Fido to make it from the drawing board to its first enemy kill. Fido claimed 33 Axis submarines in the Atlantic (32 German, one Japanese), and four more in the Pacific (all Japanese).
Fido was, in one sense, the progenitor of today's advanced air-dropped anti-submarine torpedoes, the Mk 46, the Mk 50 Barracuda, and the Mk 54 MAKO. Such is the legacy of a torpedo that sniffed out Axis subs.