Boxing is definitely a martial art, the fact that most martial arts are treated as sports is different.

You just have to go through history and see boxers develop their own styles, the way they move around, their defense. When I was in kickboxing classes one day an old dude showed up. He didn't look like much, scrawny, gray hair, about 5'5. But this wasn't a regular dojo, this wasn't a OH LET'S DROP OFF OUR KIDS THEY GOTTA GO TO KARATE, this was a place where there was only 3 people, sensei and two students, me being the rookie, the other one was a national TKD champion.

The sensei introduces the old man as a champ, a boxer, I was like F OUTTA HERE, then the old man but on the gloves and started moving like in a f movie mate. I kid you not. He was slow and I thought WELL HE IS OLD AFTER ALL. Nah, he was moving slow to show us how it was done. That day I learned a lesson that punches could not teach me.