Throughout 1942 the design was steadily refined and improved, in direct consultation with pilots who were actually flying them in combat. Improved armour protection and a fifth ShKAS machine-gun was installed and fuel tanks modified. Despite anecdotal reports by Soviet fliers, Pe-2s were daylight bombers, often crewed by comparative novices in the early years of the war, and took significant losses, even when well protected by fighters. In December 1942 General Turkel of the Soviet Air Force estimated the life expectancy of a Pe-2 was 30 combat flights. An example of loss rates after the Soviets gained the upper hand can be gained by the losses suffered by the 1st and 2nd BAK. The former started the month of July 1943 with 179 machines, and lost 52 that month, and 59 the next, ending August with 156 bombers after receiving replacements. The 2nd BAK started July with 122 Pe-2s, with monthly losses of 30 and 20, ending August 1943 with 114 Pe-2s after replacements arrived. Most of these losses were at the hands of the thinly stretched German fighter groups, which continued to inflict significant losses when present in strength, even in the closing months of the war. For example, in the Baltic where JG54 Grünherz were the main opposition, and greatly outnumbered, the Soviet 1st Gv BAK lost 86 Pe-2s shot down (another 12 to other causes), mostly to German fighters between 23 July 1944 and 8 February 1945. Western sources use mark Pe-2FT for production series after 83 (where FT stands for Frontovoe Trebovanie (Frontline Request)), although Soviet documents do not use this identification. Final versions Pe-2K (transitional version of Pe-2I) and Pe-2I were produced in small numbers, due to the unwillingness of Soviet industry to decelerate production numbers.