The competition to design a new skyscraper in Saudi Arabia had a simple but lofty rule: The tower had to be the world's first kilometer-tall building.

The design submitted by Chicago architects Adrian Smith and Gordon Gill — a sleek glass tower built atop three legs and tapering into a skyscraping needle — was announced as the winner Tuesday, further cementing Chicago's role as a hub of cutting-edge architecture.

Construction of the $1.2 billion building, called Kingdom Tower, is expected to begin around December in the waterfront city of Jeddahand be completed a little more than five years later, Smith said. The tower's exact height hasn't been revealed, but it will be the first building to be at least a kilometer, or 3,280 feet, tall, according to Adrian Smith + Gordon Gill Architecture.

If completed, the tower will be at least 563 feet taller than the world's current tallest building, the 2,717-footBurj Khalifa in Dubai, United Arab Emirates, and nearly double the height of Chicago's Willis Tower, including its antennas.

Smith was the lead designer of Burj Khalifa and Chicago's Trump International Hotel & Tower while he was with the Chicago office of Skidmore, Owings & Merrill. Burj Khalifa opened in January 2010.

The expectations surrounding record-setting buildings are often hard to meet. Burj Khalifa had relatively high vacancy rates when it opened just after Dubai's real estate bust, and others have not been built despite high-profile plans. That's true of the 2,000-foot Chicago Spire. Ireland's Garrett Kelleher promised to build it at 400 N. Lake Shore Drive, but the plan collapsed, leaving a hole at the site and millions in lawsuits from his creditors.

But judging buildings such as Burj Khalifa and Kingdom Tower on their first few years of operation is unfair because doing so fails to account for their long-term effect on the value of surrounding properties and their performance over time, said Antony Wood, executive director of the Chicago-based Council on Tall Buildings and Urban Habitat.

"It's expensive to build a supertall building," Wood said. "If you look at the history of the world's tallest, it's never been about just making the largest financial return. It's about other things. It's about ego, attention, status and all those things."

Kingdom Tower will house a luxury hotel, apartments, condos, office space and the world's highest observatory. It is part of Kingdom City, a development in Jeddah expected to cost $20 billion, according to the architecture firm.

Smith and Gill's firm beat at least five others, including Skidmore, that submitted plans for the tower. Smith and Gill said the building's backers wanted the structure to be a symbol of Saudi Arabia's future, so they designed it to resemble a plant shooting skyward.

"We looked to a powerful symbol of life springing from the ground and forming itself as a vertical spire toward the heavens," Gill said. "We used symbols such as new plant growth coming out of the sand and spoke to that as a kind of rejuvenation of intellectual capital, business and culture. I think that those are some of the things that we hope this building will come to represent."

The building's tapered, sloping shape is designed in part to counteract wind forces, building on lessons learned from the construction of some of the world's other tallest skyscrapers, Smith said.

"The design of a supertall building is very new territory in the science of structural engineering and architecture," he said. "You only get one or two of these per decade, so each time we do a supertall building, the learning curve on how to do the next one is improved by about 10 percent of man's knowledge."

The winning design was announced by Saudi billionaire Prince Alwaleed bin Talal, a nephew of Saudi King Abdullah.