Written By: Ben Cosgrove

The Pentagon has become such a symbol of the U.S. military 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.

For example:

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—60 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.

Correction: The original version of this story misstated the number of years between the groundbreaking ceremony for the Pentagon and the Sept. 11, 2001 terrorist attacks. The attacks were exactly 60 years later, not 70 years later.

Building the Pentagon, 1940s

Building the Pentagon, 1941.

Thomas D. McAvoy TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

Architects and draftsmen work on plans for the Pentagon’s construction in the partially completed building in 1942.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

A massive map provides an overview of the Pentagon highway network. With a complex housing roughly 23,000 workers and 16 parking lots for over 8,000 cars, new roads to accommodate the traffic were a necessary part of the construction.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

In a May 1943 issue, LIFE noted that the exterior of the Pentagon “has a gray limestone façade, although more than half of the building’s substance is sand and gravel dredged from the bottom of the Potomac River.”

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

An officer chats with a worker by one of the large exhaust fans at the Pentagon, 1940.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

A woman occupies a desk in the colossal office space in the “War Building.”

Thomas D. McAvoy TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

Workers would ultimately complete seven floors for the Pentagon: five of them above the ground and two beneath.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

The Pentagon, 1941.

Myron Davis-TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

When construction began on September 11, 1941, LIFE reported, the groundbreaking took place “only two weeks after the designing [of the structure] commenced.”Building the Pentagon, 1940s

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

The Pentagon was built in a mere 16 months for approximately $83 million.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

Among the more random facts about the Pentagon: the building contains an estimated 4,200 clocks.

Myron Davis-TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

The Pentagon boasts 17.5 miles of hallways.

Myron Davis-TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

The Pentagon has 284 rest rooms.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

Today the Pentagon is surrounded by 200 acres of lawn.

Thomas D. McAvoy TIME & LIFE Pictures/Shutterstock

Men at work inside the Pentagon, 1941.

Myron Davis TIME & LIFE Pictures/Shutterstock

The Pentagon’s mail room.

Myron Davis TIME & LIFE Pictures/Shutterstock

A serviceman talks to a receptionist in the newly constructed Pentagon in 1941. Building the Pentagon, 1940s

Thomas D. McAvoy TIME & LIFE Pictures/Shutterstock

Sending files via the Pentagon’s pneumatic tube system—an old-school delivery mechanism that, as late as the mid-1980s, was still handling more top-secret information than the Defense Department’s computers.

Myron Davis TIME & LIFE Pictures/Shutterstock

The official War Office seal on the china used in a private dining room at the Pentagon.

Myron Davis TIME & LIFE Pictures/Shutterstock

Part of the suite for the highest ranking officer at the Pentagon, circa 1942. As LIFE wrote in a December issue that year, the Secretary of War “has a roomy, carpeted office with comfy overstuffed leather chairs. He sits at the handsome desk which has been inherited by every Secretary of War since Robert Todd Lincoln in 1883. At his right is a direct wire to the White House.”

Myron Davis-TIME & LIFE Pictures/Shutterstock

A private kitchen built to serve the highest ranking Pentagon officials and their guests, should they wish to avoid one of the building’s six cafeterias.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

Part of the suite for the Secretary of War. LIFE wrote in December 1942: “The only really happy person in the War Department’s whopping new reinforced-concrete ‘home’ is the Army’s civilian chief, Henry L. Stimson.” (Stimson was Secretary of War at the time. This post would later be eliminated when the Army and Navy were split into separate departments; the job of Secretary of Defense was added to ensure cooperation between them.)

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

A man presses a button in the elevator reserved for the highest ranking officer at the Pentagon and his guests. The Pentagon boasts 13 elevators, 19 escalators, and 131 stairwells.

Myron Davis TIME & LIFE Pictures/Shutterstock

Building the Pentagon, 1940s

From the private washroom that was part of the suite for the Pentagon’s top man. LIFE noted in May 1943: “There is a medicine chest, toilet, and a stall shower but no bathtub.”

Myron Davis TIME & LIFE Pictures/Shutterstock

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