Neither B350 nor X370 provide their own PCIe 3.0 lanes, only 2.0 lanes (6 vs. 8). As far as I know PCIe 3.0 is always wired into the CPU directly. Please correct me if I’m wrong there though, I always read it this way.
/edit
see chart here:
The Chipset doesn’t provide PCIe 3.0, this is not Intel that somehow gets x16 lanes through an x4 PCH 
Unfortunately I don’t have a block diagram for this specific board though.
/edit2
nevermind the above… found the block diagram, the second x16 slot is actually 2.0 x4 (so effectively 3.0 x2) … you’re right.. my bad, kinda silly layout but w/e. As far as I see every B350 board does it like this though… bummer