 Complete the following phrase:  Selected for production over the competing GB-2 and GB-3 due to its simpler control system and its proving more practical for bomber carriage, production of the GB-1 began in May 1943; arriving in the combat zone in September, operational use was delayed due to the limited bombload the glider imposed  a B-17 Flying Fortress bomber could carry only two GB-1s on a mission, one on a rack under each of the bombers wings  and the accuracy of the GB-1 proving to be significantly worse than that of ordinary bombs. Despite this, by May 1944 the first releases of GB-1s were undertaken. On 28 May 1944, 42 of 113 glide bombs released hit Cologne, after being released 18 miles from the Eifeltor marshaling yard in 