The Ju 87 B series was to be the first mass-produced variant. A total of six pre-production Ju 87 B-0 were produced, built from Ju 87 A airframes. Test flights began from the summer of 1937. A small number, at least three, served as conversion Cs or Es for potential naval variants. The first production version was the Ju 87 B-1, with a considerably larger engine, its Junkers Jumo 211D generating 1,184 hp, and completely redesigned fuselage and landing gear. This new design was again tested in Spain, and after proving its abilities there, production was ramped up to 60 per month. As a result, by the outbreak of World War II, the Luftwaffe had 336 Ju 87 B-1s on hand. The B-1 was also fitted with "Jericho trumpets", essentially propeller-driven sirens with a diameter of 0.7 m (2.3 ft) mounted on the wing's leading edge directly forward of the landing gear, or on the front edge of the fixed main gear fairing. This was used to weaken enemy morale and enhance the intimidation of dive-bombing. After the enemy became used to it, however, they were withdrawn. The devices caused a loss of some 20–25 km/h (10-20 mph) through drag. Instead, some bombs were fitted with whistles on the fin to produce the noise after release.