Students began training on twin-engine Beechcraft AT-11 Kansan bomber trainers. By January 1942, there were 50 aircraft on base, in addition to 28 B-18A Bolo bombers used for training. Eventually, about 150 AT-11s served the school. The USAAF established a new minimum proficiency standard for bombardier trainees in 1943. Trainees were required to hit their targets during at least 22 percent of their drops. Practice combat flying missions required continuous evasive action within a 10-mile radius of the intended target. The final approach was required to be straight, level and taking no longer than 60 seconds.