The blaze broke out about 6:30 p.m. CST Monday and burned until nearly midnight in the LaSalle National Bank building, built in 1934.
WBBM-AM, Chicago, said at least 34 people had been taken to area hospitals, 12 of them firefighters, with eight in serious condition.
Heat from the fire shattered windows and sent sheets of glass crashing to the street as flames flickered out of the windows.
More than a third of the city's firefighting force was on the scene, department spokesman Larry Langford said. Officials appealed to suburban departments, asking them to report to the Chicago Fire Academy to be deployed throughout the city. Eleven engines and 11 trucks came from suburbs including Oak Park, Des Plaines and Naperville, the most fire departments ever called in from outside city limits, the Chicago Tribune reported.
There was no immediate cause given for the fire.