SAN FRANCISCO— Intel chief executive Paul Otellini is retiring in May, giving the world’s largest maker of ­microprocessors six months to find a new leader as it confronts two major challenges: a shaky economy and a shift toward­ mobile devices that is reducing demand for its PC chips.

Although Otellini’s impending departure was announced Monday, he notified Intel Corp.’s board of his retirement plan Wednesday. The decision surprised Intel’s board of directors, which had been expecting Otellini to remain CEO until the company’s customary retirement age of 65. Otellini is 62.

‘‘The decision was entirely Paul’s,’’ an Intel spokesman said. ‘‘The board accepted his decision with regret.’’

Otellini will be ending a nearly 40-year career with ­Intel, including an eight-year stint as CEO by the time he leaves. He joined the company after graduating from the nearby University of California Berkeley and worked his way up before succeeding Craig Barrett as CEO in 2005.

‘‘It’s time to move on and transfer Intel’s helm to a new generation of leadership,’’ Otellini said in a statement.

In another statement, Intel chairman Andy Bryant praised Otellini for leading the company through ‘‘challenging times and market transitions.’’

Otellini and the four other men who have been Intel’s CEO during the company’s 45-year history have all been promoted from within.