Which cryptanalyst co-developed the Bombe with Alan Turing?

Gordon Welchman co-developed the British Bombe with Alan Turing.

The Bombe was an electromechanical machine used at Bletchley Park during the Second World War to help identify the settings of German Enigma machines. Turing produced the initial British design in 1939, building on earlier Polish cryptanalytic work and the Polish bomba. In 1940, mathematician and cryptanalyst Gordon Welchman introduced an important improvement known as the diagonal board.

Welchman’s refinement made the Bombe far more powerful by allowing it to exploit additional relationships among Enigma settings. The machine did not simply read messages automatically; it tested possible settings using a likely fragment of plaintext, called a crib, and eliminated settings that produced logical contradictions.

The first British Bombe, code-named Victory, was installed in March 1940. Later machines, including Agnes, incorporated Welchman’s improvement. Harold Keen and the British Tabulating Machine Company handled much of the engineering and construction, so “co-developed” appropriately recognises the different contributions of Turing, Welchman, and the engineering team.