This article examines engineering and military ethics in the Japanese Navy during World War II. It focuses on what a group of engineers did from 1944 to 1945 for research and development of MXY 7, the world's only aircraft specifically created and deployed for suicide missions. Contrary to common portrayals of the military operation, I argue that the process was complicated and contested, especially among the engineers. They remained neither monolithic nor fanatic, rationally debating inherently moral issues within themselves. The engineers' detailed calculations, design blueprints, and personal memos during the war testify to this untold story of the war.