AMD really has not released any information on B550 motherboards. For the mid-range we instead had the launch of B550A, which was more or less a refreshed B450 for system integrators. The actual B550 series has never been confirmed by AMD, but we’ve heard that board partners have been working on boards for a long time.
The big difference between B450 and B550 is that B550 will have PCI-Express 4.0 support. B450 actually did technically support PCIe 4.0, but AMD removed this option with an AGESA update. B550A only supported PCIe 4.0 with the primary slot connected directly to the CPU. The actual B550 chipset is expected to support PCIe 4.0 on all slots.
The first picture of a B550 motherboard has been leaked to the web and it comes from SOYO. Looking at the board it is micro-ATX with a brown PCB, red PCIe slots, and a blue VGA connector. There are two Gen4 x16 slots and a smaller x4 slot as well as a single Gen4 M.2 slot. We can also see a cool dragon graphic on the board itself.
No word on when we will actually see B550 motherboards for sale.