The upper end of a rotary drill bit having initially retracted expansible cutters is connected to a stabilizer having initially retracted expansible stabilizer members, the upper end of the stabilizer being connectable to a rotary drill pipe string. The apparatus is lowered in a bore hole on the drilling string and rotated, the cutters being expanded to cut into the bore hole and enlarge it to a desired diameter, the apparatus being lowered to progressively continue enlarging the hole in a downward direction. Upon the stabilizer moving into the enlarged hole, its stabilizer members are expanded laterally to an extent conforming to the enlarged hole diameter, being engageable with the wall of the bore hole to stabilize the bit therebelow.