Colossus, built in 1943, was a computer that had a fixed function: to break coded messages. The same was true of the ENIAC (Electronic Numerical Integrator And Computer), developed around the same time.
Changing the program of either of these computers involved rewiring the machine and physically pulling switches. The first practical general-purpose stored-program electronic computer was EDSAC (electronic
delay storage automatic calculator), which ran its first program in 1949. It could be reprogrammed with ease, and typically worked for 35 hours a week, carrying out calculations that a human would find complex and time-consuming.
The developers of the ENIAC went on to design and build the EDSAC, the first computer that stored programs and data in the machine.