Retail giant Wal-Mart has been named the largest company in the world in the latest Fortune 500 list. The Arkansas-based firm, which employs 1.2 million people worldwide, moved up a place from the number two spot to take over at the top from oil giant Exxon Mobil. The coveted list is based on companies' annual revenue figures, with Wal-Mart posting $219.8 billion in revenue last year. US car giant General Motors claimed third spot.