Image of Irvin McDowell and George McClellan via Wikipedia
The Department of Northeastern Virginia was created on May 27, 1861, and Brigadier General Irvin McDowell was placed in command of its army.
McDowell was responsible for the area of Virginia in front of Washington. He was expected to take the raw volunteers that were streaming in, the first units to respond to Lincoln's call for militia after Fort Sumter, and somehow organize them into an army. Before he could do this, the government and popular opinion prodded him into an advance on General P. G. T. Beauregard's Confederate Army of the Potomac.
The Army of Northeastern Virginia was routed at the First Battle of Bull Run on July 21, 1861. After the defeat, on July 25, the Department of Northeastern Virginia was abolished and merged into the Military Department of the Potomac, then formed the nucleus of Major General George McClellan's Army of the Potomac.
Prior to the First Battle of Bull Run, the Army of Northeastern Virginia was the largest ever assembled in American history, some 30,000-35,000 men. Many of these early recruits were signed to three-month enlistments, limiting McDowell's options. Some units' enlistments ran out on the eve of the battle, and they returned home to muster out rather than fight.
The Army of Northeastern Virginia consisted of five divisions led by Brigadier General Daniel Tyler, Colonel David Hunter, Colonel Samuel Heintzelman, Brigadier General Theodore Runyon, and Colonel D. S. Miles. Runyon's Fourth Division was held in reserve.
There was also a unit from the United States Army Corps of Engineers and another from the United States Topographical Engineers, a balloon detachment.