The one million dollar bill is not real, and it only exists as a novelty item. These novelty bills are fun to show off and give as gifts. But of course they are not legal tender. Since this isn't a real bill, it's not worth anything. Novelty one billion dollar bills also exist.
There is no official design to these faux million dollar bills and they come in different types. There are bills with historical figures, cartoon characters, etc. However most bills have Lady Liberty on the obverse side of the bill. Click here to search for different million dollar bills on Amazon.
The US will likely never issue legitimate bills of this denomination. But it's interesting to note that legal tender million dollar bills do exist in the country of Zimbabwe. They were printed because the country experienced hyperinflation and the currency lost its purchasing power.
The largest US bill ever issued for circulation was the 10,000 dollar bill. A 100,000 dollar bill dollar bill was created but it was never put into circulation. Large bills were mainly used between banks for large transactions.