In his youth, Bruce Lee studied Wing Chun under the tutelage of Yip Man and his "older brothers" of the art, such as Wong Shung Leung or Cheung Cheuk-Hing (William Cheung). Furthermore, young Lee also studied Tai Chi Ch'uan with his father and, although sporadically, other styles of Gung Fu, like Hung Gar, Choi Li Fut or Ching Wu, to mention some. Bruce was a member of Hong Kong crews among which this mixture of styles was usual, having to face opponents with different fighting systems.
At the age of 18, Bruce went to the USA to keep American citizenship and after making some money as a Cha-Cha teacher over there, he started forging a career as a Wing Chun instructor, a new style in America those days. Bruce wanted to create, in his own words, "a superior method of Gung Fu" and, therefore, the teachings of his schools were based on what he called "Jun Fan Gung Fu Method" (Lee Jun Fan was his Chinese name), that consisted on a personal modification of Wing Chun.
On December, 1964, something happened that would change Lee's martial approach: he was challenged by a recognized Chinese Gung Fu practitioner just arrived from Hong Kong. Bruce defeated him in barely 3 minutes. Despite winning the combat, Lee felt frustrated; he got too tired and felt the fight should have finished sooner. He started seeing limitations in his "modified Wing Chun" and two were the main consequences after that fight: to improve his fitness to endure a combat in full condition and to acquire a footwork that would allow him to keep the desired distance in every moment of the fight. That was the germ of what he would later call "Jeet Kune Do".
For more information: Bruce Lee Foundation