The Pentagon — and, by extension, the U.S. military — has become such a prominent and obvious symbol of American might over the years that it's easy to forget that the world's largest office building and home of the Department of Defense is just that, a building, albeit one with some mighty impressive stats, and some sobering history, attached to it.
— Despite having 3,705,793 square feet for offices, concessions, and storage and a gross floor area of 6,636,360 square feet, the Pentagon is designed so that, ideally, it takes at most seven minutes to walk between any two points in the building.
— Five-and-a-half million cubic yards of earth and 41,492 concrete piles were necessary for the foundation of the building, as well as 680,000 tons of sand and gravel from Potomac that were processed into 435,000 cubic yards of concrete.
— Roughly 200,000 telephone calls are made daily from the Pentagon. (In the pre-cell phone days of the early 1940s, 100,000 miles of telephone cable — enough to circle the globe four times — helped make all that communication possible.)
— Ground was broken for construction on the Pentagon on September 11, 1941 — 70 years to the day, incidentally, before one of the airliners hijacked by terrorists on 9/11, American Airlines flight 77, slammed into the western side of the building, killing 184 people, 125 of them in the Pentagon itself.
The people who actually worked inside the Pentagon, meanwhile, were initially underwhelmed by the building, LIFE wrote in December 1942. Both employees and visitors "resent the eight and two-fifths miles of barren corridors, the jammed ramps, the pile-up at entrances and exits, the parking and transportation problems, the six overcrowded cafeterias, the staggered working hours."
Here, LIFE.com presents a series photos — most of which never ran in LIFE — of the iconic, colossal edifice under construction more than seven decades ago.
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter @lizabethronk.