W.M. Owen's In Camp and Field with the Washington Artillery
speaks of using red flannel strips worn by his unit as
identification at 1st Bull Run.
Afterward, Philip Kearney had his men of the 3rd Division, III Corps wear a red diamond.
When Joe Hooker assumed command of the Army of the Potomac he prescribed that all his corps be identified.
Daniel Butterfield is credited with designing the patches. The following year (1864) corps badges appeared in the western theater.