The 8031 is an MCS-51 series microcontroller, brought to you by your friends at intel. It is in the same family as the 8051. The only difference is that the 8051 has internal PROM, whereas the 8031 does not. The other member of the family is the 8751, which has internal EPROM. There are also CMOS versions of all, as well as many clones not made by intel.