 He and Iwerks would go on to develop a new cartoon in secret, starring a new character which would soon become the most successful cartoon character in film history and later became the foundation of a global entertainment empire.The first Mickey Mouse cartoon to be filmed was "Plane Crazy" in the summer of 1928, but it was produced as a silent and held back from release.The first Mickey Mouse film with a synchronized soundtrack, "Steamboat Willie", reached the screen that fall and became a major hit, eclipsing Oswald.