Tommy Flowers was a telephone engineer at the General Post Office research station at Dollis Hill when he proposed building an electronic machine to crack the Lorenz cipher used by the German High Command. The codebreakers at Bletchley Park were skepticalâthey didn't believe vacuum tubes (valves) could be reliable enough. Flowers used his own savings to help fund the initial development.
Colossus Mark 1, completed in December 1943, used 1,500 vacuum tubes and could process encrypted text at 5,000 characters per second. The improved Mark 2, with 2,400 tubes, was rushed into service just in time for D-Day in June 1944, providing crucial intelligence about German defensive preparations and confirming the success of deception operations.
After the war, Flowers was told to destroy all documentation and the machines themselves. His pioneering work remained classified for decades, meaning he received no credit for building the world's first programmable electronic computer. When he approached banks for loans, citing his computing expertise, they didn't believe him. He received only ÂŁ1,000 from the British government for his wartime contributionâfar less than his personal financial investment in the project.