Konrad Zuse

 
Defining one point along this road as "the first digital electronic computer" is exceedingly difficult. On 12 May 1941 Konrad Zuse completed his electromechanical Z3, being the first working machine featuring automatic binary arithmetic and feasible programmability (therefore the first digital operational programmable computer, although not electronic); other notable achievements include the Atanasoff-Berry Computer (shown working around Summer 1941), a special-purpose machine that used valve-driven (vacuum tube) computation, binary numbers, and regenerative memory; the secret British Colossus computer (demonstrated in 1943), which had limited programmability but demonstrated that a device using thousands of valves could be both made reliable and reprogrammed electronically; the Harvard Mark I, a large-scale electromechanical computer with limited programmability (shown working around 1944); the decimal-based American ENIAC (1946) — which was the first general purpose electronic computer, but originally had an inflexible architecture that meant reprogramming it essentially required it to be rewired.