The Federal Reserve, in a dramatic effort to rev up a "disappointingly slow" economic recovery, said it will buy $600 billion of U.S. government bonds over the next eight months to drive down interest rates and encourage more borrowing and growth.

Many outside the Fed, and some inside, see the move as a 'Hail Mary' pass by Fed Chairman Ben Bernanke. He embraced highly unconventional policies during the financial crisis to ward off a financial-system collapse. But a year and a half later, he confronts an economy hobbled by...