A $1 million bill is a fake no matter what it looks like. The largest-denomination bill printed by the U.S. Treasury was the $100,000 1934 Gold Certificate. These certificates were used for official transactions between Federal Reserve banks and did not circulate.
A Canadian firm once produced nonnegotiable "One Million Dollar" platinum certificates in a special limited art series and sold them for $1. They are not official currency and are not redeemable. The Secret Service has determined that having them is not a crime, so anyone who has one can enjoy it without fear of arrest, according to the U.S. Department of the Treasury.