A drill bit assembly for fluid-operated percussion drill tools. The assembly comprises a percussion bit comprising a bit head detachably connected to an axially extending shank and engagement means formed on an external wall of the shank engageable with complementary engagement means formed internally of a drive chuck whereby rotational drive from the chuck may be transmitted to the shank. At least one flushing channel extends between the external wall of the bit shank and a cutting face of the bit head, to provide a fluid path for a flushing medium between the external wall of the bit shank and the cutting face of the bit head.