The B/X Companion is an attempt to create the missing third volume in the 1981 version of the "Basic" Dungeons & Dragons line. Both Tom Moldvay's Basic Set and Dave Cook and Steven Marsh's Expert Set (together known as the "B/X" edition) referred to the third box, which was supposed to cover characters from levels 15 to 36.

When the two existing box sets were rewritten in 1983, the new author (Frank Mentzer) took the line in a different direction, creating three new sets that bore little resemblance to the "Companion Set" promised in the original Expert Rulebook (the Companion box, for instance, only covered levels 15 to 25).

The B/X Companion uses the format and hints provided in the B/X box sets. The topics covered are similar to those in Menzter's official Companion, Masters, and Immortals box sets, but the specifics are different: 70 new spells (up to 7th level for clerics and 9th level for magic-users), 100 new magic time, new abilities for characters of 15th and higher level, and rules for running a dominion, rules for mass combat, and rules for siege warfare (based on Swords & Spells).