Well, because we also love searching and finding the answer for this kind of stuff, here’s how it goes.
The Pentagon was built in 1941 when the US was nervously watching Nazi Germany aggressively move all over Europe. At the time, the almost 24,000 War Department employees were spread out across 17 different buildings located in the district of Columbia. The site that was given for the new offices was a five-sided parcel of land that was almost adjacent to the Pentagon’s current site. (the article continues after the ad)
The chief of the design section, major general Hugh John Casey, was tasked to come up with a building that would best fit the area’s unique shape. In order to maximize the site’s odd dimensions, the designers and the architects came up with a pentagonal plan with each of five “wedges” connected through corridors.
However, because some people were concerned that the new building would block the sweeping vistas view, President Franklin D. Roosevelt decided to relocate the offices on the current site – on the 15th Street bridge, at the foot of the Virginia side.
Of course, the new plot of land didn’t have the same limitations but the architects, noting the design’s many advantages, stuck with their original pentagonal plan. As chief historian in the Office of the Secretary of Defense, Erin R. Mahan explains: “(it would) permit easier access from one part of the building to another than could be achieved with a more conventional square or rectangular layout.”
In addition, in order for the building not to obstruct the views, the whole structure couldn’t be more than 4 stories high. To have 40,000 people in just 34 acres (137,000 sq.m.), pentagon was the perfect shape and it also required very little steel, something which was important at the time, since this precious material was very much needed for building ships and weapons.
And that is why the pentagon is designed as a pentagon!