
The 1940 U.S. Federal Census is the largest and most comprehensive record set available that records the names of those who were living in the United States at the time the census was taken. The 1940 United States Census included several standard questions, such as: name, age, gender, race, education, and place of birth. But the census also introduced some new questions like where people lived 5 years before, highest educational grade achieved and detailed income and occupation information.
Currently, the Census is not indexed by name. To find individuals, you would need to know the location of their residence or the enumeration district number. The National Archives provides some help on how to begin your search. You can also help index the Census through the Community Indexing Project.