 Their fuel tanks were also well protected by layers of self-sealing rubber, although the incendiary and tracer ammunition which was carried by RAF fighters could sometimes ignite fuel vapour in empty tanks.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.