It was only a matter of time.

With ETFs that track broad equity indexes trading more than most individual stocks, and investors pouring money into fixed-income funds, it probably should come as no surprise that someone decided to marry the two. We give you: the S&P 500 Index bond fund.

The ProShares S&P Bond ETF, ticker SPXB, started trading on Thursday. The latest fund from $29 billion ProShares, the tenth largest issuer of ETFs, aims to give investors exposure to the most liquid and high quality bonds issued by companies in the S&P 500, according to the fund’s prospectus.

While broad market trends continue to push money into fixed-income ETFs, some investors are still hesitant to trust debt instruments that trade like stocks, according to Michael Sapir, chief executive officer of ProShare Advisors LLC, the parent of ProShares funds. The familiarity of the S&P 500 name, though, could now help lure people off the sidelines, he said.

Of the almost 5,000 investment-grade bonds issued by S&P 500 companies, the fund’s index selects up to 1,000 of the most liquid notes. Debt with a maturity at first issuance of under 2.5 years or a par value of less than $750 million are excluded. Those that are left are then ranked in descending order by liquidity as measured by the average trading day volume over the previous 60 days, according to fund documents.

Bonds issued by Wall Street banks like Morgan Stanley, Goldman Sachs Group Inc., Citigroup Inc. and JPMorgan Chase & Co. -- as well as Apple Inc. -- are among the fund’s top holdings, according to the ProShares website.