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.