William was the son of Robert I, Duke of Normandy by his concubine Herleva. He was born in Falaise, Normandy in 1027 or 1028. William became the Duke of Normandy when his father died in 1035. In 1034 or 1035 Duke Robert wanted to go on a pilgrimage to Jerusalem. He made his noblemen swear to make his young son William their duke if he was killed.