The He 111 was nearly 100 mph slower than the Spitfire and didn't present much of a challenge to catch, although the heavy armour for the crew stations, self-sealing fuel tanks and progressively uprated defensive armament meant that it was still a challenge to shoot down. It was the most numerous German bomber type during the Battle, and was capable of delivering 2000 kg of bombs to the target, carried in an internal bomb bay – usually eight 250 kg bombs, stored vertically. Subsequent variants allowed further increase in the bomb load and the maximum size of bombs carried, with external bomb racks. The state-of-the art "Lotfernrohr 7" gyroscoping bomb sight fitted to the Heinkel allowed for reasonable accuracy, for a level bomber. The main versions of the He 111 in use were the Jumo engined H-1, H-2 and H-3 and the DB 601 powered P-2 and P-4. Small numbers of the aircraft, called H-1x and H-3x, were equipped with "Knickebein" and "X-Gerät" and were used by Kampfgruppe 100 (KGr. 100) at night during the closing stages of the battle. "Y-Gerät" equipped H-5y of III. Gruppe Kampfgeschwader 26 began to take part in the Blitz of the winter of 1940–1941.