Historical data for US corporate tax rate is harder to find. However, you want to consider the difference between the statutory rate (35% now), and the actual effective rate (appx 25% now). Many corporations pay far less than the statutory rate, due to various tax avoidance strategies. The 1986 tax reform act closed many loopholes but many others have since emerged.
The statutory rate is easy - it was lowered from 46% in 1987 (and prior years) to 40% in 1988 and 34% in 1989. It went to 35% in 1994, and has been there ever since.
The effective tax rate has changed a lot more, due to economic cycles and evolving tax avoidance strategy. In gneral, it has been going down. This is a chart.
Here is a discussion of that difference.
http://www.cbpp.org/10-16-03tax.htm