The main difference between the E and F was that wide blade propellers were fitted to the new type that gave better performance. The F was made in greater numbers and had to be manufactured by three different companies; each factory had slight detail modifications. The source factory was listed by a suffix placed after the designation and block number: BO stood for Boeing, VE for Lookheed Vega and DL for Douglas. Thus, a plane with the designation B-17F-109 VE told you that it was made by Lookheed.
The B-17F's arrived in England in August 1942 and were destined to fly throughout 1943. However by the summer of 1944 they were a rare sight on operational bases. The planes were subsequently re-modifed by the idividual bases to cope with the unique problems that com to light in air combat. The aircraft, designed and tested in warmer climes, had to cope with the extremly low temperatures and high humidity of altitude flight. Problems encountered in the first few missions: the brushes in the electrical generators frozen up, the ball turret would not rotate, guns jammed, there was blind spot in the forward zone of fire and the tail was very heavy.
"To find out at the beginning as us tried as one attacks the B-17 best, tried we bomb to and so on everything, even the bombers bombs. But we found out that the best tactics consisted in attacking her from the front and we used the 190s for it as end of '43. The time at which you could shoot was very short since the approach-speed was very high. But, when you have hit the B-17 from the front, you have hit the cockpit or the engines mostly. There were only four 190s groups after this time which attacked from behind, which were called the "storm troops". If the B-17 didn't burn or the garrison didn't jump down, then these 190s rammed the bombers at the tail unit or the rudder."