Thomas (Tommy) Harold Flowers, MBE (22 December 1905 – 28 October 1998) was a British engineer who designed Colossus, the first digital, programmable, electronic computer. Flowers was born in London. After an apprenticeship in mechanical engineering, he earned a degree in electrical engineering at the University of London. In 1926, he joined the telecommunications branch of the General Post Office (GPO), moving to work at the research station at Dollis Hill on the northwest side of London in 1930. From 1935 onward, he explored the use of electronics for telephone exchanges. By 1939, he was convinced that an all-electronic system was possible. This background in switching electronics would prove crucial for his computer design in World War II.

In 1942, Flowers was posted to Bletchley Park, 50 miles (80 km) northwest of London, to join the code breaking efforts there. He worked on breaking a teletype-based cipher called "Geheimschreiber" (secret writer) by the Germans and "Fish" by the English decoding team that was much more complex than the German's Enigma system. The decoding procedure involved trying so many possibilities that it was impractical to do by hand. In February 1943, Flowers proposed an electronic system (Colossus) using 1500 valves (vacuum tubes). Because the most complicated previous electronic device had used about 150 valves, some were skeptical that such a device would be reliable. Flowers countered that the British telephone system used thousands of valves and was reliable because the electronics were operated in a stable environment that included having the circuitry on all the time. Years later, Flowers described the design and construction of these computers. With the highest priority for acquisition of parts, his extremely dedicated team built the first Colossus computer in 11 months. It operated 5 times faster and was more flexible than the previous system, code-named Heath Robinson, that used electro-mechanical switches. Anticipating the need for additional computers, a redesign utilizing 2400 valves was begun before the first computer was finished. The Mark 2 operated 5 times faster than the first Colossus. Flowers estimated that they could be manufactured at a rate of about one per month.

Ten Colossus machines were completed and used during World War II in British decoding efforts, and an eleventh computer was ready for commissioning at the end of the war. All but two of the Colossus computers were dismantled at the end of the war. "The remaining two were moved to British secret service headquarters, where they may have played a significant part in the codebreaking operations of the Cold War" (Nova, 1999). The final two were decommissioned in 1959 and 1960.

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