Remind Me Again: Friedrich Paulus

The Nazis planned to push four armies forward and only four armies forward in the summer and fall of 1942--Sixth, Seventeenth, First Panzer, and Fourth Panzer. One can understand why an army personnel office would choose Hermann Hoth as commander of Fourth Panzer Army and von Kleist as commander of First Panzer Army. But Richard Ruoff as commander of the Seventeenth Army? And Friedrich Paulus as commander of the Sixth Army? What was there in their previous careers to mark them as the right people for army command in Southern Russia in the summer and fall of 1942?