Wall Street's biggest firms can't get a break in the bond business.

With trading profits dwindling, more dealers than ever are fighting for assignments managing U.S. corporate-bond sales, one of the few bright spots in fixed income. Companies from the most-creditworthy to the most-indebted have been selling trillions of dollars of debt, locking in record-low borrowing costs ahead of the anticipated rise in interest rates.

The increased competition is bad for JPMorgan Chase & Co., Bank of America Corp., Citigroup Inc., Goldman Sachs Group Inc., and Barclays Plc because the top five banks won the smallest share of the assignments this year for any comparable timeframe, according to data compiled by Bloomberg. A record 144 underwriters for the period have split an estimated $4.2 billion of fees on U.S. sales, the data show.