The time period of 1945 to 1955 is considered as First Generation of Computers.
Around 1945, Howard Aiken at Harvard, John Von Neumann at Princeton, J Eckert and William Mauchely at the University of Pennsylvania and K Zuse in Germany succeeded in designing calculating machines with vacuum tubes as the central components.
These machines were huge and their continued usage generated a great deal of Heat.
Note about these computers :
1) No assembly Language nor any higher Language
2) Single User
3) Unfriendly to user