If you have a box truck that is cdl rated and you remove the box and make it a flatbed is it still a cdl truck?

This question is a bit asinine, if I can be blunt here. Yes, of course it is. CDL requirements are determined primarily by the weight rating of the vehicle. If it's a tandem truck rated at 54,000 lbs., then that's how its rated, whether it has a box, flatbed, mixer, dump body, etc. If it's a single axle truck rated at 33,000 lbs., more the same story. You MIGHT be able to register it as a 26,000 lb. GVWR vehicle, in which case, it won't require a CDL anymore - single axle trucks can be a bit ambiguous that way.