Board Thread:General Discussion/@comment-6746577-20131127231527/@comment-8986886-20131128033047

In ch.390 (ep.293), Gin explains that the reason the Espada followed Aizen, before he obtained the Hōgyoku, was that they feared his individual strength far more than his Kyōka Suigetsu's Shikai ability; which means Aizen certainly had the potential to achieve Bankai.

If you also account for his intelligence, and how he was able to master Zanjutsu, Kidō, Shunpo and Hakuda, I'd say the chances of Aizen not having achieved Bankai are slim to none.