In July 1862, Kimball led his regiment, the 8th Ohio Infantry, and the 7th West Virginia Infantry to join the Army of the Potomac. After adding the 4th Ohio Infantry to his brigade, Kimball was briefly stationed in Fort Monroe before covering John Pope's retreat from the Second Battle of Bull Run on September 1. During the subsequent Maryland Campaign, Kimball commanded the First Brigade of William H. French's third division in the II Corps. His men formed the right of the division during its series of assaults on the Sunken Road at the Battle of Antietam, losing over 600 men killed or wounded. Despite the severe losses, the men held their ground and eventually pushed aside the Confederates in their front, capturing 300 men and several colors. Kimball's brigade became known as the "Gibraltar Brigade" for their steadfast ability to withstand enemy fire.