There once was a developmental league for the NFL, but it was in Europe. The goal of his league was only partially for developmental purposes, it was also meant to expand the interest in the NFL around the world. It was a flop and is now defunct, mainly because Europe didn't want the NFL's sloppy seconds.
The idea to peddle second rate football to enhance interest in the NFL across the ocean in Europe might have failed, but the development of players, coaches, and officials in the league did show some dividends. Now there are more and more people that believe that the NFL actually needs a developmental league, just not one all the way in Europe.