Colossus

Built by telephone engineer Tommy Flowers and brought into service at Bletchley Park in December 1943, two years before ENIAC. Its target was not Enigma but the Lorenz cipher, used for German high command traffic. Colossus read paper tape at a thousand characters per second and counted statistical coincidences to find the cipher's wheel settings.

Flowers paid for part of the build out of his own pocket after superiors dismissed the idea that vacuum tubes could be reliable enough. Ten machines were running by the end of the war. Churchill ordered them destroyed and the drawings burned; the secrecy was not lifted until the 1970s, which is why computing history counted ENIAC as the first for three decades.