The defensive armament originally consisted of a single ShKAS machine gun in a dorsal TSS mounting, later MV-3 turret, manned by the navigator in the cockpit behind the pilot, and a ventral 7.62 mm ShKAS machine gun in a mounting in the fuselage behind the bomb bay aimed by a periscope. This defensive armament was found to be totally inadequate: from Series 13 onwards, the ventral ShKAS was replaced by one 12.7 mm UBT in an MV-2 mounting, while the dorsal ShKAS was also replaced by a UBT in various different mountings, some modified in the field and others from the factory: