Looks like Williams FW20 or 21.
Apparent absence of barge boards seem to point to 1999 FW21.
Then again, the sidepod and flipups look like the FW20.
The "M" on the helmet looks like Jorg Muller.
I don't think I've seen this pic before. There was an article on the Williams-BMW test mule in BMW Magazine way back when, but the car was white with IIRC blue pinstripes.
Edit: Sorry didn't answer your question. Car looks like a Williams-BMW test mule pre-2000. Obviously Jorg Muller is BMW's factory driver. I could be wrong about the white car though, because there was also a Williams/Michelin test car that was white.
Here you go: Jorg Muller in a FW21B, Michelin tyre testing. Click for more.