FORTUNE — A few rows of airline seats are bolted to the floor in a room with sweeping views of the North Carolina countryside at Lenovo’s U.S. Headquarters in Morrisville. It’s an incongruous sight, until you realize that if you are going to make one of the most enduring tools for business travelers, well, you need to make sure it’s going to work well on airplanes.
The room is a showcase for the ThinkPad, the iconic black-box notebook computer that IBM (IBM) first introduced in 1992. Twenty-one years and dozens of models later, Lenovo, which bought IBM’s PC business in 2005, is still making ThinkPads, which are instantly recognizable as descendants of that original model. Indeed, the ThinkPad is not only one of the bestselling business laptops of all time, but also the most enduring design in the history of the computer industry.
Much of the responsibility for the ThinkPad’s slowly evolving look falls on David Hill, 56, a soft-spoken Oklahoman, who joined IBM in 1985. He took over design of the ThinkPad line in 1995, as IBM was considering how to update a three-year-old computer that had become a mega-hit.
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“The question was, What are we going to do now?” Hill says of the time when he joined. “My view was that if it is not broken, why fix it?” The ThinkPads were selling so well that they had sparked a string of copycats. “So why not make it better?” He asked.
Hill and his team of designers in North Carolina and Japan have been doing that ever since.
The first ThinkPad was conceived by Richard Sapper, a renowned German-born designer. Sapper envisioned a device that would have the simplicity of a cigar box or a bento box. The process was not without hiccups. When Sapper chose a particular kind of red for a pointing stick that would be embedded into the keyboard to control the cursor, IBM’s product safety group balked. Red, apparently, could only be used for emergency shutoff buttons, says Hill.
Sapper quickly came up with a workaround. He varied the tone a bit a created a new color that he called “IBM magenta.” After the new color got the okay from the product safety police, he gradually reverted the tone back to the red he had originally chosen, and no one seemed to notice. Nowadays, the red dot is an integral part of the ThinkPad design, and one that still elicits strong reactions: Users either love it or hate it.
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Over the years, ThinkPads got faster, slimmer and lighter, of course. But their design, while staying true to the original concept, also embraced innovation. Among the most iconic models was the ThinkPad 701c, unveiled in 1995. It was a 10-inch box whose “butterfly keyboard” expanded when users opened the machine, allowing them to type on a full-size keyboard. It became part of the design collection at New York’s Museum of Modern Art. The ThinkPad was also the first laptop to come with a pop-up second screen, and the first to have a detachable base so users could travel without the extra weight of a CD drive.
Hill is fond of highlighting design details like the shape of the ThinkPad’s keys, which, rather than being a square shape like most computer keys, have a curved bottom edge. Hill says that makes the keyboard more forgiving of mistakes, a claim that has been validated by third-party tests. His favorite machine, not surprisingly, is his latest, the X1 Carbon, an ultrathin, three-pound laptop with a 14-inch screen that Laptop magazine called the best business Ultrabook.
“We are working on a huge number of ideas to take ThinkPad to new places,” says Hill. In the past year, two Lenovo tablets got the ThinkPad name, which in a sense is a return to the past, since before launching the first ThinkPad notebooks, IBM released a pen-based ThinkPad tablet that was too early for its time. (It’s not impossible that a ThinkPad phone could be in Lenovo’s future.) “Design is a moving target, and we are still evolving it,” he says. “But we have a foundation, a fan base.” Hill plans to make sure that no matter how much the ThinkPad evolves, something in it will still evoke the bento box that Sapper conceived more than two decades ago.