Catholic doctrine teaches that Peter was the first Pope and that Popes may speak "ex cathedra". When they do, that is considered infallible.
However, while Peter was alive and presumably serving as the first Pope, Paul, John, and James all wrote books that are considered to be Scripture. In fact, Peter himself refers to Paul's letters as Scripture:
and regard the patience of our Lord as salvation; just as also our beloved brother Paul, according to the wisdom given him, wrote to you, 16 as also in all his letters, speaking in them of these things, in which are some things hard to understand, which the untaught and unstable distort, as they do also the rest of the Scriptures, to their own destruction. 2 Peter 3:15-16 NAS
Were other people besides the Pope, then, able to speak/write ex cathedra while not serving as Pope? If so, can others besides the Pope speak/write ex cathedra today? What is the Catholic understanding of this?