"XB" just means "Expert Built". Basically, it's Tamiya's way of saying it's a roller. Best to build your own kit, so you'll know better how to fix it.
Several Tamiya models are "XB". Most of the on-road ones are a TT-01. TT-01 is not a race chassis. There are tons of hop-ups for it...that still won't make it a competitive race chassis, but it will cost as much as one.
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XRAY T4'15 x2 | XRAY XB4 | XRAY XB8E | XRAY XB8