In the spring of 1928, Disney traveled to New York City in hopes of negotiating a more profitable contract with his producer Charles Mintz. But as economic problems were apparent at the time, Mintz figured Disney should settle for a 20% cut, although large turnarounds were promised if the studio's finances showed considerable growth. While most of his fellow animators left for Mintz's studio, Disney decided to quit working on the Oswald cartoons. On his long train ride home, he came up with an idea to create another character, and retain the rights to it. 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. "Plane Crazy" was later given its own synchronized soundtrack and released on March 17, 1929.