The time machine went through several variations. In the first draft of the screenplay the time machine was a laser device that was housed in a room. At the end of the first draft the device was attached to a refrigerator and taken to an atomic bomb test. Robert Zemeckis said in an interview that the idea was scrapped because he and Steven Spielberg did not want children to start climbing into refrigerators and getting trapped inside. The Nevada desert bomb test was left out in order to reduce the budget. In the third draft of the film the time machine was a DeLorean, but in order to send Marty back to the future the DeLorean had to be driven into an atomic bomb test.
(Source: filmtrivia, via jarrodmatthew)
