The 3rd Regiment Massachusetts Volunteer Heavy Artillery was organized by Special Order No. 1087 on September 8, 1864. The 13th Company Unattached Massachusetts Volunteers had been previously mustered into service on February 10, 1864. Company I, of the 13th Unattached, was recruited largely from mechanics employed at the National Armory in Springfield, Massachusetts, and as such, was employed as an engineer corps during the terms of service. Company I was commanded by Captain John Pickering of Salem. The company was assigned to duty under Captain F. N. Farquhar, U. S. Corps of Engineers, and was placed in charge of the pontoons of the Army of the James. They were the last men of the regiment to be mustered out on September 26, 1865. The 15th Company Unattached Massachusetts Volunteers, Company L, was organized on May 30, 1864 and commanded by Captain Joseph M. Parsons of Salem. The company performed garrison duty in the defense of Washington, D. C. The company was mustered out on September 18, 1865.

This series is arranged into two subseries. Subseries A. 13th Co. Unattached, Heavy Artillery, Company I includes monthly and quarterly returns related to clothing, camp, and garrison equipage, along with general and special orders, statements, and correspondence that discusses pensions for members of the 13th Unattached. Subseries B. 15th Co. Unattached, Heavy Artillery, Company L includes one volume of clothing accounts for the 15th Unattached.