Microsoft Corp. is outshining Apple Inc. in at least one area: bond sales.

The software giant on Monday completed the largest U.S. corporate-bond sale this year, selling $10.75 billion in debt with maturities of five to 40 years. The sale surpasses a pair of megabonds issued last week, an $8 billion sale by drug maker Merck & Co. and a $6.5 billion deal from electronics titan Apple.

Investors...