The original Fortune 500 was limited to companies whose revenues were derived from manufacturing, mining, and energy exploration. At the same time, Fortune published companion "Fortune 50" lists of the 50 largest commercial banks (ranked by assets), utilities (ranked by assets), life insurance companies (ranked by assets), retailers (ranked by gross revenues) and transportation companies (ranked by revenues). Fortune magazine changed its methodology in 1994 to include service companies. With the change came 291 new entrants to the famous list including three in the Top 10.
Via fortune.com & Wikipedia.org