I dug up at the US Census Bureau serveral reports about family and individual income and created a series of graphs plotting the income distribution of households under $100,000 a year adjusted for inflation. (Pages 17, 18, 19 from my Income Guide)
I am defining middle-income households as $30,000-$80,000. One of the stories these graph tell is that for 20+ years after 1945 more households entered the "middle class". However, over the next 40 years, the percent of middle-income households shrank in part because the percent of households with more than $80,000 a year grew.
Graphs created in OmniGraphSketcher and annotated in Illustrator. Data from the US Census. You take a look at some of the older reports they have online here:
US Census Bureau. “Families and Individual Money Income in the United States: 1945. Table 2.” September 2011. http://www2.census.gov/prod2/popscan/p60-002.pdf.
———. “Income of Families and Persons in the United States: 1950. Table 1.” September 2011. http://www2.census.gov/prod2/popscan/p60-009.pdf.
———. “Income of Families and Persons in the United States: 1960. Table 5.” September 2011. http://www2.census.gov/prod2/popscan/p60-037.pdf.
———. “Income, Poverty, and Health Insurance Coverage in the United States: 2010. Table A-2.” September 2011. http://www.census.gov/prod/2011pubs/p60-239.pdf.