Mechanical and electrical computing machines have been around since the 19th century, but the 1930s and 1940s are considered the beginning of the modern computer era.

• The German Z3 (shown working in May 1941) was designed by Konrad Zuse. It was the first general-purpose digital computer, but it was electromechanical, rather than electronic, as it used relays for all functions. It computed logically using binary math. It was programmable by punched tape, but lacked the conditional branch. It was destroyed in a bombing on Berlin in December 1943.
• The American Atanasoff–Berry Computer (ABC) (shown working in summer 1941) was the first electronic computing device. It implemented binary computation with vacuum tubes but was not general purpose, being limited to solving systems of linear equations. It also did not exploit electronic computing speeds, being limited by a rotating capacitor drum memory and an input-output system that was intended to write intermediate results to paper cards. It was manually controlled and was not programmable.
• The British Colossus computers (used for cryptanalysis starting in 1943) were designed by Tommy Flowers. The Colossus computers (of which ten were built) were digital, all-electronic, and could be reprogrammed by rewiring, but they were dedicated to code breaking and not general purpose.^[19]
• Howard Aiken's 1944 Harvard Mark I was programmed by punched tape and used relays. It performed general math functions, but lacked any branching.
• The ENIAC was, like the Z3 and Mark I, able to run an arbitrary sequence of mathematical operations, but did not read them from a tape. Like the Colossus, the operations happened at electronic speed. The ENIAC combined full, Turing complete programability with electronic speed.

The ABC, ENIAC and Colossus all used thermionic valves (vacuum tubes). ENIAC's registers performed decimal arithmetic, rather than binary arithmetic like the Z3 or the Atanasoff-Berry Computer.

Until 1948, ENIAC required rewiring to reprogram, like the Colossus. The idea of the stored-program computer with combined memory for program and data was conceived during the development of the ENIAC, but it was not implemented at that time because World War II priorities required the machine to be completed quickly, and it was realized that 20 storage locations for memory and programs would be much too small.