As of August 2014, in the Bleach anime series, it has not yet been revealed what the Bankai of Kisuke Urahara may be. It is preserved as a mystery throughout the series.
Kisuke Urahara is the founder and first president of the Shinigami Research and Development Institute, and the former captain of the 12th Division in the Bleach series. He owns a small convenience store called Urahara Shop in the human world, where he is in exile. He sells Shinigami items at his shop alongside several of his employees. A rumor appears early in the series that he learned Bankai within three days through the use of a specific technique, but he refuses to train others with this method, or to reveal his Bankai. He states that his Bankai is not one that can be used to train others. Bankai, or “Final Release,” is the second and final upgraded form of a Zanpakutō.
To attain Bankai, the ability to materialize and subjugate a Zanpakutō spirit is needed. Materialization is the ability to not be forced into the Zanpakutō's inner world. This skill can take 10 years or more to achieve. A Zanpakutō is the most advanced weapon of the Shinigami, Arrancar, and Visored in the Bleach series.