The United States million dollar bill is a fake denomination of currency - the U.S. never printed million dollar bills. The largest legal bill ever made was the one hundred thousand dollar bill, which was used for transactions between federal reserve banks, and was never actually put into public circulation.
There ARE companies who print millions as novelty items, usually bearing a picture of the Statue of Liberty on the front. These companies are occasionally investigated by the U.S. Secret Service, but are not usually prosecuted.
And, even though these bills are not legal currency, once in awhile someone will try to spend one as cash - in 2004 a woman was arrested for forgery for trying to use one at a Walmart in Covington, Georgia.