BMW secretly made a convertible E34 M5 and Z3 Roadster powered by a V-12

To celebrate 50 years of the M division, BMW has been showing off once-secret prototypes in a series of videos, but the two shown in the latest video released Wednesday are among the most unusual yet. Once upon a time, BMW built an E34 M5 convertible and a Z3 M Roadster prototypes with V-12 engines. Both cars are now part of an M 50th anniversary…