Aon Center was originally named the United California Bank Building from its completion in 1973 until 1981, when it became First Interstate Tower. It was the tallest building west of the Mississippi River when built, until 1982 when it was surpassed by the Texas Commerce Tower in Houston. Upon its completion in 1973, the building was the tallest in the world outside of New York and Chicago. It remained the tallest building in Los Angeles until 1989, when Library Tower (now U.S. Bank Tower) was completed. Between 1998 and 2005, there were no logos on the building.
On May 4, 1988, a fire began on the 12th floor just after 10:00 PM; it burned for about four hours. The fire destroyed five floors, injured 40 people, and left a maintenance worker dead because the elevator opened onto the burning 12th floor. The fire was so severe because the building was not equipped with a sprinkler system, which was not required for office towers at the time of its construction. A sprinkler system was 90 percent installed at the time of the fire; however, the system was inoperative, awaiting the installation of water flow alarms. The fire was eventually contained at 2:19 AM, and caused $400 million in damage. Repair work took four months. Because of the fire, building codes in Los Angeles were modified, requiring all high-rises to be equipped with fire sprinklers. This modified a 1974 ordinance that only required new buildings to contain fire sprinkler systems, grandfathering older buildings. Existing all-concrete construction high-rises are still exempt from this ordinance.
The north entrance is level with 6th Street, and is named BL (Bank Level since a Wells Fargo Bank branch occupies the eastern half of that floor). The east and west sidewalks slope downward to Wilshire Blvd. with steps leading up to the south entrance. Elevators on the south side of BL and escalators on the north side of BL both go up to the ML (Main Lobby) level, where additional banks of elevators reach floors numbered 4-62. No 2nd floor exists, though the height of ML is twice that of ML (hence, this is a 62-story tower with only 61 floors). The BL/ML elevator also goes down to underground levels LBL (Lower Bank Level), LL1 (Lower Level 1 with evacuation tunnel used by firefighters in 1988), and LL2 (valet parking garage).