Within just a few years of the Auditorium's completion in 1908, the student body had already outgrown it, and planning for a new Armory began. As one proposal for a new Armory noted, "there is no building in the University where half [crossed out and says one-third] of the students [s crossed out] body can be assembled for any purpose. The Auditorium, but recently completed, cannot [crossed out] accommodate half of the [crossed out and now says only about 2000] students." 
In 1912 the Georgian Revival-style Armory was born, a 152,600 square foot structure  built at a cost $229,119.17.   Designed by W. Zimmerman,  the centerpiece of the structure was a 200 by 394 foot drill floor,  reaching a center height of 98 feet,  with spectator galleries seating 3,000. The entrances to the galleries were strategically placed in the towers so that audience members would not have to cross onto the drill floor at any time.  Fourteen massive three-hinged arches,  weighing 36 tons each, support the center of the structure. 
The Armory was largely used by the University's cadet regime, which by 1912 was the largest in the country. All underclassmen were required to train for three hours a week, and a total of 1,525 soldiers trained actively, double that of most other National Guard units.  The building also provided housing for more than 2,000 soldiers during World War I.  By 1935, the New Armory, along with the other military-related buildings on campus, housed more than $2,000,000 worth of United States Army equipment and resources. 
Due to its huge size, the Armory became a "general exhibition and assembly hall", the first such structure in the University's history capable of holding under one roof farmer's conventions and other large assemblies. 
Additions and renovations to the Armory were performed in 1927,  when James White and Charles Platt designed a $425,000 addition, and 1963,  when the second story balconies that once adorned the building's four corner towers were removed. 
By 1911 the land immediately south of the Armory was known as the Military Drill Field and measured 900 by 2200 feet, stretching from modern Gregory to Peabody Drives on the North and South and Sixth to First streets on the West and East.  By 1915 it occupied 16.5 acres and was known as the Parade Ground. 
On April 20, 1920, 173 memorial trees were planted around the Armory in what was then known as Military Drill Field, circling the entire block and stretching along Sixth Street for a block and a half. Each tree is dedicated to one University of Illinois student who died in World War One. There is still a map in the University Archives that lists the name honored by each individual tree. 
By the 1950's the Armory housed "an indoor rifle range, an indoor running track, and room for baseball practice indoors".  In 1967 the University approved an $11,920 remodeling of the Armory.