On May 9, 1941, during an attack on Atlantic convoy OB 318, U-110 under Kapitänleutnant Fritz-Julius Lemp was forced to the surface by depth charges from HMS Bulldog, HMS Broadway, and the corvette HMS Aubretia. Lemp ordered his crew to abandon ship and set scuttling charges, but the charges failed.

Sub-Lieutenant David Balme led a boarding party from HMS Bulldog onto the still-functioning U-boat. Inside, they found a treasure trove: a naval four-rotor Enigma machine with its rotors in the current settings, the short-signal codebook used for weather reports and contact sightings, the officer's cipher manual, and current encryption keys. The material was transferred to Bulldog while Lemp, who realized the capture, apparently tried to swim back to the submarine and drowned.

The captured materials provided Bletchley Park with the tools to read German naval communications for months. To protect the secret, the Admiralty spread the story that U-110 had sunk—which it eventually did, under tow. The capture was kept secret until the 1990s.