Johnson reported to the group's 61st Fighter Squadron on July 19, 1942, in Bridgeport, Connecticut. The unit had just received the first production P-47B Thunderbolts, and, in effect, was flight testing the new fighter as it trained. While the 56th FG was responsible for many of the modifications that made later variants a successful fighter-bomber, the training resulted in more than forty crashes and 18 fatalities, many of which Johnson blamed on the inadequacy of the small airport at Bridgeport. However, he also asserted that many more lives would have been lost, had not the P-47 proved to have an exceptionally rugged airframe. The P-47 became the first USAAF aircraft to provide an understanding of compressibility and its effects.