It's hard to believe Chicago started out as a small trading post at the mouth of the Chicago River. It incorporated as a city on March 4, 1837, and grew exponentially in the decades that followed. Most of its buildings were made of wood. During the dry summer of 1871, a massive fire broke out, destroying more than 17,000 buildings across Chicago and killing 300 people. The cause of the fire was never determined. This photo shows the fire's devastation at the corner of State Street and Madison Street. Editor's note: An earlier version of this gallery had an image of downtown Chicago that was removed because its authenticity was in question. After further review with the source, CNN found the image was misrepresented as an historical photograph. It was in fact a still from the 1937 movie "In Old Chicago." CNN regrets the error.