Leader of the ENIAC project, Brainerd was perhaps most famous for his work leading the development of the world's first large-scale electronic digital computer. During World War II, the differential analyzer at the Moore School was used intensively for ballistic computations under contract to the Army Ordnance Department. It became evident to Brainerd that if this work was to continue at its rate of growth, the work would outrun the capability of the facility.
It was in this atmosphere of pressure that led to the inception of the ENIAC (Electronic Integrator and Calculator) project in 1946, the world's first large-scale electronic digital computer. It was under Brainerd's inspiration, leadership, and supervision that the ENIAC was conceived and built.
In addition to Project ENIAC, he was supervisor of an Automatic Data Retrieval System which provided large-scale team consulting services to the Signal Corps on what was then the largest interconnected computer system yet conceived.
