The primary defensive arrangement for the standard series production SB consisted of twin 7.62mm ShKAS machine guns mounted in the navigator’s nose cockpit. These weapons were characterized by an exceptional rate of fire for the era, reaching 1,800 rounds per minute. In standard configurations, such as the SB-2-M-100-A and SB-2-M-103, each nose-mounted gun was supplied with 860 rounds of ammunition.

Technically, the installation allowed for a vertical travel of 180° and an initial horizontal traverse of 15° to each side. In later production blocks, specifically starting from the 17th series, the horizontal traverse angle was increased to between 23° and 25° to provide a wider field of fire. The navigator operated these weapons using a PMP-3 gun sight, and the design allowed the guns to be fired either singly or in unison. A persistent operational issue involved cold air entering the nose compartment through the large machine gun clearance slots; this was eventually addressed by installing movable Plexiglas sheets that sliding along guide rails to seal the openings.