Junkers Ju 87 armoured protection of the Ilyushin Il-2 "Sturmovik" - a feature pioneered by the 1916-17 origin Junkers J.I all-metal sesquiplane of World War I Imperial Germany's Luftstreitkräfte - was copied to protect the crew from ground fire now that the Ju 87 would be required to conduct low level attacks. Hans-Ulrich Rudel, a Stuka ace, had suggested using two 37 mm (1.46 in) Flak 18 guns, each one in a self-contained under-wing gun pod, as the "Bordkanone BK 3,7", after achieving success against Soviet tanks with the 20 mm MG 151/20 cannon. These gun pods were fitted to a Ju 87
Junkers J 2 earlier J 1's structure. Junkers began wind tunnel and design work promptly upon receipt of the contract, and by the end of the spring of 1916 the first example was completed. The J 2 differed from the J 1 in having a cowling that almost entirely enclosed the engine, a rounded upper and lower fuselage section instead of the rectangular section of the J 1., and a narrower and deeper ventral radiator enclosure, and had a horizontal stabilizer planform shape that would become familiar on later, all-duralumin Junkers monoplane designs to be built during 1917-18. The "all-moving" rudder still possessed