The Blenheim Mk IV was not, as is often thought, a variant of the Mk I. Rather it was based on the Type 149 variant, based on the 11/36 requirement for a coastal defence bomber. During flight trials the Mk IV reached the top speed of 266 mph which was better than the Mk I. However the data was misleading, as the speed was reached at 12,000 ft, whereas the Mk I was tested at sea level, where it reached 239 mph. Obviously the Mk I was slightly faster at altitude. Unfortunately, this became obvious in combat, when Mk IVs fared no better against Luftwaffe fighters than the Mk I. It quickly became clear that Blenheims needed fighter escorts.