B-1s started rolling off the lines in December 1941, but they were delivered at a trickle. In preparation for the new plane, I./SchlG 1 had been formed up in January with Bf 109 E/Bs (fighter-bomber version of Bf 109 E) and Hs 123s, and they delivered B-0s and every B-1 that was completed. Still, it was not until April that 12 B-1s were delivered and the 4th "staffel" (squadron) became ready for action. They moved to the Eastern Front (to Crimea) in the middle of May 1942, and in June they received a new weapon, the 30 mm (1.2 in) MK 101 cannon with armor-piercing ammunition in a centerline pod.