We can default on maturing bonds any time the Executive Branch sees fit. All they have to do is to refuse to honor those bonds with the cash available on hand. Of course right now, several times more cash comes flowing into the Treasury each week than there are bonds becoming due, not to mention that those same bonds can be immediately resold without increasing the debt one dime. And don't forget the $21 billion per week that the Fed continues to dump into our Treasury in spite of the debt ceiling.
So if a default occurs, it will be intentionally brought about as part of this Administration's 'scorch-America' policy.