technological advances. Wing boxes Although the combat box was initially designed around a group of airplanes, it was expanded to include a formation of three groups flying together as a "combat wing", all of which were based on the same triangular design of a leading bomber (or grouping of bombers) in the center, and two bombers (or groupings) immediately behind in a vee shape, with one at an altitude above and one below the center in close proximity for mutual defense. The new combat boxes continued to be used, but groups were placed in a horizontal column and stacked at increasing altitude to decrease their vulnerability to attack. This resulted in the rear formations lagging behind which impacted both defensive tactics and bombing performance. The wing box, a 54-plane formation (basically three 18-plane boxes stacked in a similar fashion to the group box), evolved from a need to provide defensive fire against head-on attacks. The formations were stacked so that while squadrons were stacked away from the sun on missions, groups were stacked in the opposite direction, making for a more compact wing box. Because the Eighth AF had only four B-17 units until May 1943, composite groups were formed by combining squadrons from different groups, or a fourth group was added to the wing box to form a diamond-shaped box. The latter was flown in a trailing position behind the wing and proved susceptible to Luftwaffe tactics of attacking the extremities of formations. The wing box was often spread vertically, deep, and across and was difficult to maintain. The most serious disadvantage of the wing box was that the lowermost and uppermost elements, trailing at the end of the formation, had the least mutual protection. An additional risk was that of an aircraft being struck by falling bombs if it strayed in the box (see image): <blockquote>"Directly below us (on 19 May 1944 to Berlin in B-17G 42-102411<ref>Miss Lace</ref>) in the path of our falling bombs was a B-17 out of position. Later pictures show a B-17 having his left stablizer shorn off by a five hundred-pound bomb dropped from above. That plane went into a steep dive, out of control, and was lost. As mentioned before, the bombs had to fall about five hundred feet or more before the protective vane spun off to leave the bomb armed." —303rd BG pilot Richard Riley Johnson</blockquote> In the summer of