Zuse began his work on program-driven calculating machines in 1935. His two predessors of the Z3, the Z1 and Z2, were unsuccessful mechanical calculating machines. The Z3 was delivered to the Deutsche Versuchsanstalt für Luftfahrt (German Experimental Department of Aeronautics) in Berlin and was used for deciphering coded messages. A 1960 reconstruction of the Z3 is in the Deutsche Museum in Munich.
Of the potential rival claimants to the title of first programmable computer, Babbage (UK, c1840) planned but was not able to build a decimal, programmable machine. Atanasoff's ABC, completed in 1942 was a special purpose calculator, like those of Pascal (1640) and Leibniz (1670). Eckert and Mauchly's ENIAC (US), as originally released in 1946, was programmable only by manual rewiring or, in 1948, with switches. None of these machines was freely programmable. Neither was Turing et al.'s Colossus (UK, 1943-45). Aiken's MARK I (1944) was programmable but still decimal, without separation of storage and control.