ENIAC

Completed in 1945 at the University of Pennsylvania by John Mauchly and J. Presper Eckert for the US Army. The machine held around 18,000 vacuum tubes, weighed thirty tons, and performed roughly 5,000 additions per second — a thousand times faster than the electromechanical calculators it replaced.

It had no stored program. Switching it to a new calculation meant rewiring cables and setting switches, work that could take days and was carried out by six women — Betty Snyder, Jean Jennings and their colleagues — whose contribution was for decades described as operating rather than programming. The insight that the program belonged in memory was formulated shortly afterwards and became the von Neumann architecture.