Google officials agree there is more competition, but say they are not simply reacting to the younger search engine.

Google’s new features have not been in response to Bing, said Marissa Mayer, the company’s vice president for search products and user experience. “A lot of these things have been in the works for a long time,” she said. “Left-hand navigation we worked on for almost two years. We wanted to make sure we had it exactly right.”

Microsoft’s gains are far from staggering. Its share of searches has grown to 12.7 percent, from 8 percent, since Bing was introduced in May 2009, and Yahoo, which has a search deal with Microsoft, still handles a larger share of searches than Bing. And in the newest search frontier, mobile devices, Google has even more market share than on the Web at large.

Still, Bing’s gains have impressed analysts, who have watched Google fend off repeated assaults on its lucrative search and ad business, which accounts for some 95 percent of its revenue.

Building a more comprehensive, faster and more accurate search engine than Google is a daunting challenge, and a long list of big companies and start-ups have failed in their attempts. Microsoft endured plenty of ribbing as it spent years building and then scrapping search systems meant to help it compete against Google. But it kept experimenting until it found a way.

Image When Microsoft introduced it last year, Bing made a splash with its vivid background images. In June, Google presented searchers the option of a colorful background rather than the stark, white page.

Microsoft has spent billions of dollars building the computing centers needed to power search and advertising systems and acquiring start-ups with niche expertise. In addition, it has thrown money at consumers, through cash-back programs on purchases, and at partners willing to promote Bing ahead of Google. Over the last year, Microsoft’s online services division lost $2.36 billion on revenue of $2.2 billion.