The Congressional Budget Office has released Historical Effective Federal Tax Rates: 1979 to 2003. Here is the Introduction:

The following tables update the series of historical effective tax rates estimated by the Congressional Budget Office (CBO) by providing values for an additional calendar year: 2003. The tables show effective tax rates for the four largest sources of federal revenues—individual income taxes, social insurance (payroll) taxes, corporate income taxes, and excise taxes—as well as the total effective rate for the four taxes combined. The tables also present average pretax and after-tax household income; counts of households; and shares of taxes, income, and households for each fifth (quintile) of the income distribution and for the top percentiles of households.