Fannie Mae rolled out the 40 yr mortgage in 2005. As the market continued to trend upwards, with homes becoming prohibitively more expensive, our nation's Financiers found new ways to keep the payments down. One of the lesser-used mortgages was the 40 yr Fixed. It is simply that: a Principal and Interest Mortgage with the payments amortized over 40 yrs. Most borrowers in a situation where they needed to keep costs down would simply choose a 10/20 mortgage, an Adjustable Mortgage, or an Option ARM. 

So the 40 yr never enjoyed the success of some of its counter parts. Recently (2010), the 40 yr has been all but phased out, as its interest rates have reached a point where its monthly payments exceed that of a 30 yr mortgage and make it a non-viable option.