Well, one of the first will be for sure Intels System 310/286. According to the CHM this would be 1981, but I think it was rather 1982/83 - that would also fit the original flyer. It's a Multibus system using the iSBC 286/10 single board computer running Xenix. The board was also used by several other manufacturers for early 286 based Unix sytems and real time applications - way until the 1990s - Intel even added a revised iSBC 286/10A in 1985 to satisfy demand. Here a nice report about someone reviving a 310. When searching the net for more information, be aware that the system 310 was also/originaly available with a 8086 board.