The He 111Z Zwilling was a design that entailed the merging of two He 111s. The design was originally conceived to tow the Messerschmitt Me 321 glider. Initially, four He 111 H-6s were modified. This resulted in a twin-fuselage, five-engine aircraft. They were tested at Rechlin, and the pilots rated them highly. The He 111 Z-2s and Z-3s were also planned as heavy bombers carrying 1,800 kg (3,970 lb) of bombs and having a range of 4,000 km (2,500 mi). The ETC extensions allowed for a further four 600 L (160 US gal) drop tanks to be installed.