After Vogt returned to Germany, Doi became the key person in the design bureau of Kawasaki Aircraft until the company ceased operations at the end of World War II His most important and outstanding work was the design of Army Type 3 Fighter Kawasaki Ki-61 ""Hien"". Ki-61 "Hien" demonstrated surprising performance that surpassed the famous Mitsubishi A6M Zero, World War II fighter equipped by the Imperial Japanese Navy Air Service. A total of 3,159 Ki-61 "Hien" and its variants were built. As mentioned above, designer of A6M Zero was Jiro Horikoshi, a classmate of Doi at university. After WWII, Doi was forced to interrupt work on aircraft design until the Treaty of San Francisco came into force in 1952. During this period, he worked as a day laborer, while continuing to have faith that he would eventually return to aviation field. He continued to study the latest technology with Kimura who was the professor fired from the University of Tokyo. Kimura was Doi's best friend. When the treaty lifted the ban on designing and operating aircraft, Doi returned to his original work. In 1956 the Ministry of International Trade and Industry of Japan announced a domestic production plan of middle-sized commercial aircraft, i.e. the YS-11. The consortium of companies, the Nihon Aircraft Manufacturing Corporation (NAMC), was established. NAMC included Mitsubishi Heavy Industries, and Fuji Heavy Industries, Shinmeiwa Manufacturing, Japan Aircraft, Showa Aircraft, and Kawasaki Heavy Industries (Kawasaki Aircraft). Doi was nominated the chief designer of equipment team. Doi assumed the professor to Meijo University after he retired Kawasaki Heavy Industries. He also serve on a councilor of the Japan Society for Aeronautical Science, a trustee of Japan Aeronautic Association, and an advisor emeritus of Kawasaki Heavy Industries. Details of his philosophy on aircraft design were written in his memoirs, regarding the designing policy, developmental history of the aviation technology, and his friends, published in Japan in 1989.