U.S. investment firms are readying the first line of exchange-traded funds designed to give American investors access to China's swelling onshore bond market, which has been largely closed off to foreigners.

At least four fund managers - Deutsche Bank AG, Global X Funds, KraneShares and Van Eck Global - have outlined plans to launch China onshore bond ETFs, according to company filings with the U.S. Securities and Exchange Commission. The first of the funds could launch as early as this month, sources familiar with the matter say.