Boeing Would Be Biggest-Ever US ‘Fallen Angel’ If Cut to Junk

If cut to junk status, Boeing Co. will be the biggest US corporate borrower to ever be stripped of its investment-grade ratings and join junk bond indexes, flooding the high-yield market with a record volume of new debt to absorb.

On Tuesday, S&P Global Ratings said it’s considering downgrading the planemaker to junk as strikes at its manufacturing sites persist, hurting production. Last month, Moody’s Ratings said it’s considering a similar move. Fitch Ratings has highlighted the growing risks but not yet announced a review.

Downgrades to junk from two of Boeing’s three major credit graders would leave much of ...