[2] In that year when presenting the plan for a computer with 2,000 electron tubes, Zuse and Schreyer, who was an assistant at Wilhelm Stäblein's [de] Telecommunication Institute at Technische Universität Berlin, were discouraged by members of the institute who knew about the problems with electron tube technology.

"[10]: 102  In 1940, Zuse and Schreyer managed to arrange a meeting at the Oberkommando der Wehrmacht (OKW) to discuss a potential project for developing an electronic computer, but when they estimated a duration of two or three years, the proposal was rejected.

The Z2 was completed and presented to an audience of the Deutsche Versuchsanstalt für Luftfahrt ("German Laboratory for Aviation") in 1940 in Berlin-Adlershof.

[26] Zuse moved on to the Z4 design, which he completed in a bunker in the Harz mountains, alongside Wernher von Braun's ballistic missile development.

When World War II ended, Zuse retreated to Hinterstein in the Alps with the Z4, where he remained for several years.

"[13] This seeming limitation belies the fact that the Z3 provided a practical instruction set for the typical engineering applications of the 1940s.

Mindful of the existing hardware restrictions, Zuse's main goal at the time was to have a workable device to facilitate his work as a civil engineer.

[10]: 113, 152  In 1937, Claude Shannon introduced the idea of mapping Boolean algebra onto electronic relays in a seminal work on digital circuit design.

Zuse's coworker Helmut Schreyer built an electronic digital experimental model of a computer using 100 vacuum tubes[29] in 1942, but it was lost at the end of the war.

[33][34] The Colossus (1943),[35][36] built by Tommy Flowers, and the Atanasoff–Berry computer (1942) used thermionic valves (vacuum tubes) and binary representation of numbers.

[citation needed] The ENIAC computer, completed after the war, used vacuum tubes to implement switches and used decimal representation for numbers.

In this they implemented the stored-program concept which is frequently (but erroneously) attributed to a 1945 paper by John von Neumann and colleagues.

[39][40] Von Neumann is said to have given due credit to Alan Turing,[35][41] and the concept had actually been mentioned earlier by Konrad Zuse himself, in a 1936 patent application (that was rejected).

[42][43] Konrad Zuse himself remembered in his memoirs: "During the war it would have barely been possible to build efficient stored program devices anyway.

"[44] Friedrich L. Bauer later wrote: "His visionary ideas (live programs) which were only to be published years afterwards aimed at the right practical direction but were never implemented by him.