Designed by architect Bruce Graham and engineer Fazlur Rahman Khan of Skidmore, Owings & Merrill (SOM), it opened in 1973 as the world's tallest building, a title that it held for nearly 25 years.

Each year, more than 1.7 million people visit the Skydeck observation deck, the highest in the United States, making it one of Chicago's most popular tourist destinations.

[14] The first option was the Goose Island area northwest of the Loop, but Sears's vice president of real estate, Matthew J. Stacom, rejected this proposal.

Bernard Feinberg, Albert I. Rubenstein, and Philip Teinowitz had assembled that site over the previous five years, but they had failed to acquire a neighboring 74,000-square-foot (6,900 m2) lot from bus company Greyhound Lines.

[19] Some floors were designed with smaller footprints to attract prospective lessees, so the building's height was increased to meet Sears's floor-area requirements.

[15][19] FAA officials publicly denied that they had imposed a height limit;[24] however, the area's minimum safe altitude would need to be raised by 1,000 feet (300 m) if the building was just 1 foot (0.30 m) taller.

In addition, contractors installed temporary generators that could supply up to 14,000 kilowatts (19,000 hp) simultaneously; during the winter, most of this electricity was used to heat the exposed steel beams on the lowest five floors.

[44] At the end of the month, the company applied for permission to increase the building's height limit by 350 feet (110 m) and install a new antenna,[45][46] although eight of Chicago's ten television stations criticized the plan.

[47][48] On May 17, 1972, Judge LaVerne Dickson, Chief of the Lake County Circuit Court, dismissed the suit, saying, "I find nothing that gives television viewers the right to reception without interference.

[62] Local television stations WTTW and WLS-TV were planning to install temporary broadcast antennas atop the tower when the steel frame was completed.

[79] Six other stations remained at the John Hancock Center, citing a study which showed that relocating to the Sears Tower would provide only minimal benefits.

[82] By March 1974, three-fourths of the space in the building was occupied; Sears had leased the upper stories to tenants such as Goldman Sachs, Northwest Industries, and Schiff Hardin.

[94] Sears nearly sold the tower to Canadian company Olympia & York, but the deal was canceled in September 1989 because the two firms could not agree on who would pay the property taxes.

The negotiations resulted in an agreement where Sears would no longer be liable for the $850 million loan, although it would only nominally own the building, while AEW and MetLife effectively had total control.

These projections were not met, with the tower facing the same vacancy and other problems it saw under Sears, although Trizec made somewhat successful efforts to attract new tenants.

[108] Since 2007, the owners had considered plans for the construction of a hotel on the north side of Jackson Boulevard, between Wacker Drive and Franklin Street, close to the entrance of the observation deck, above the tower's underground parking garage.

In March 2009, London-based insurance broker Willis Group Holdings agreed to lease a portion of the building and obtained the naming rights.

[117][118] Blackstone announced a $500 million renovation in January 2017, which would include the construction of the Catalog, a six-story commercial complex, replacing a plaza on Jackson Boulevard and the entrance on Wacker Drive.

[121][139][140] At the time, although the Willis Tower was nearly 85 percent leased, the number of tenants and visitors entering the building had decreased significantly since 2019, in part because of the COVID-19 pandemic in Chicago.

[146] In response to the perceived threat of an attack, the building's largest tenant at this time, Ernst & Young, moved to North Wacker Drive in early 2009.

[148] The Willis Tower was designed by architect Bruce Graham and structural engineer Fazlur Rahman Khan of Skidmore, Owings and Merrill.

It was both structurally efficient and economic: at 1,450 feet, it provided more space and rose higher than the Empire State Building and cost much less per unit area.

Located on the 103rd floor, 1,353 feet (412.4 m) above ground level, it is the highest observation deck in the United States[174] and one of Chicago's most famous tourist attractions.

[178] In January 2009, a major renovation of the Skydeck began, including the installation of retractable glass balconies which extend approximately 4 feet (1.2 m) from the facade of the 103rd floor, overlooking South Wacker Drive.

When the Petronas Twin Towers in Kuala Lumpur, Malaysia, was completed in 1998, it claimed to be the tallest building in the world, measuring 1,482.6 feet (451.9 m) tall including decorative spires.

[189] Upon completion, One World Trade Center in New York City surpassed the Willis Tower through its structural and pinnacle heights, but not by roof, observation deck elevation, or highest occupied floor.

However, the extension of the tower's western antenna in June 2000 to 1,729 feet (527 m) allowed it to just barely claim the title of tallest building by pinnacle height.

[191] On May 25, 1981, Dan Goodwin, wearing a homemade Spider-Man suit while using suction cups, camming devices, and sky hooks, and despite several attempts by the Chicago Fire Department to stop him, made the first successful outside ascent of the tower.

catchphrase from the American television sitcom Diff'rent Strokes[113] and considered the name-change ill-advised in "a city with a deep appreciation of tradition and a healthy ego, where some Chicagoans still mourn the switch from Marshall Field's to Macy's".

[198] This feeling was confirmed in a July 16, 2009, CNN article in which some Chicago-area residents expressed reluctance to accept the Willis Tower name,[199] and in an article that appeared in the October 2010 issue of Chicago magazine that ranked the building among Chicago's 40 most important, the author pointedly refused to acknowledge the name change and referred to the building as the "Sears Tower".