Answer
The letter "B" in the name "ABC" honors John Vincent Atanasoff's assistant, Clifford E. Berry. Berry was a physics graduate student at Iowa State University who worked closely with Atanasoff on the development of the Atanasoff-Berry Computer (ABC), the world's first electronic digital computer. Berry's contributions to the ABC included designing and building much of the computer's circuitry and assisting with the programming.