Wellpoint Inc. and Principal Financial Group Inc. are leading companies selling $28.9 billion of dollar-denominated bonds in the busiest start to a week in six months as yields touched unprecedented lows.

Wellpoint, the second-biggest U.S. health insurer, is planning a $3.25 billion, four-part bond issue and Des Moines, Iowa-based Principal is raising $600 million as offerings reach the most since $30.2 billion in the two-day period ending March 6, according to data compiled by Bloomberg. Sales have already exceeded the $13.4 billion issued during the past two weeks.

Companies rushed to market as yields on investment-grade bonds fell below 3 percent for the first time. Borrowers are coming to market ahead of a meeting by the European Central Bank, where officials are expected to provide details on a bond-buying proposal.