The KC-135R's have to go into the depot for heavy maintenance every four or five years for something like 9 months. The GAO had a report suggesting that with about 430 KC-135R and C-135R aircraft in the USAF inventory that about 100 National Guard KC-135E's could be converted to KC-135R's and just get engines with planes entering depot maintenance. That way perfectly good CFM-56 engines wouldn't just be sitting on aircraft undergoing mainentance.
And Boeing probably could have won that contract in a fair competition. By paying the bribes, they ended up tainting the whole deal.