Which English knight became the first known holder of the title Duke of Norfolk in 1397?

Thomas Mowbray became the first known holder of the title Duke of Norfolk in 1397. King Richard II created him duke after he had already inherited the title Earl of Nottingham and held the office of Earl Marshal.

Mowbray was a powerful nobleman descended from the medieval Mowbray family. His political conflict with Henry Bolingbroke, the future King Henry IV, became one of the defining disputes of Richard II's troubled reign. In 1398, the king stopped their planned judicial duel and banished both men.

Mowbray died in Venice in 1405 while participating in a failed plot against Henry IV. The title's history is complicated because it was later forfeited, restored, and recreated. Mowbray is nevertheless generally identified as the first Duke of Norfolk in the English peerage tradition.