The 2014 Tax Foundation data below demonstrate that the U.S. individual income tax continues to be very progressive, borne mainly by the highest income earners.

139.6 million taxpayers reported earning $9.71 trillion in adjusted gross income and paid $1.37 trillion in individual income taxes.
The share of income earned by the top 1 percent of taxpayers rose to 20.6 percent. Their share of federal individual income taxes also rose, to 39.5 percent.
The top 50 percent of all taxpayers paid 97.3 percent of all individual income taxes while the bottom 50 percent paid the remaining 2.7 percent.
The top 1 percent paid a greater share of individual income taxes (39.5 percent) than the bottom 90 percent combined (29.1 percent).
The top 1 percent of taxpayers paid a 27.1 percent individual income tax rate, which is more than seven times higher than taxpayers in the bottom 50 percent (3.5 percent). 
In addition, 40% pay NO Federal income tax, and the majority of them are working.