Air battles during the first year of the war clearly showed that the Pe-2's armament did not provide effective self-defense. The Pe-2's Achilles' heel was it defense against attacks from the upper rear. This area was covered by the ShKAS machine gun in the TSS-1 mount, installed in the navigator's cockpit. The TSS-1 was inconvenient and had a narrow field of fire. The ShKAS machine gun, with its rifle caliber, proved to be too weak a weapon against German fighters.
In 1942, the Pe-2FT ("FT" meaning “front-line request") was designed as a response to the front-line units' requests to significantly upgrade the aircraft's armament. The TSS-1 pivot mount was replaced with a FT turret equipped with a large-caliber 12.7mm UBT machine gun with 200 rounds, which had been designed as quickly as possible (using some parts and assemblies from the MV-2 hatch mount) by a team of Factory No. 22 designers headed by L. L. Selyakov.
The opening in the rear section of the navigator's cockpit was increased to accommodate the cumbersome FT mount and provide a good firing angle. However, the long-barreled UBT machine gun could not be folded up in a travel position in the fuselage as the ShKAS machine gun from the TSS-1 mount could. So the rear section of the canopy, the so-called "turtle," was simply removed.
The main advantage of the FT mount was the ease of its mounting on the aircraft. The airframe structure had to be altered only minimally. Introduction of the new turret had virtually no effect on the number of aircraft produced. They began to introduce the FT mount on some Pe-2s beginning with series 83, but it was only installed on all production planes as late as series 87.
The first Pe-2s with the navigator's new large-caliber machine gun saw its first operational trial near Kerch in April 1942. The new weapon immediately demonstrated its high efficiency.
Some FT mounts were manufactured in the form of factory conversion kits to be installed on earlier series Pe-2s by combat units. It was not difficult to mount the new armament; two mechanics could typically perform the installation of the new weapon in 4 to 6 hours.