The Aichi AB-1 was a result of a 1926 government call for a small, Japanese-built, civil transport biplane able to operate from land or water. It won the contest in both roles but did not reach production as airlines moved from biplanes to monoplanes. Nonetheless, it remained in commercial use well into the 1930s. Design and development In February 1926 three manufacturers responded to a funded call from the Aircraft Bureau of the Department for a Japanese built, small passenger aircraft. All three designs were single-engined biplanes and all were based on earlier designs, only one of which was Japanese. Aichi's contestant was based on the Heinkel HD 25, a two seat reconnaissance seaplane that Aichi had built as the Navy Type 2 while military aircraft construction in post-world War I Germany remained forbidden. Alterations included a new engine, the passenger cabin, wings with slightly greater span, reduced stagger and revised struttage, a new vertical tail and optional additional landplane landing gear. The AB-1 had a wooden structure which was covered with a mixture of plywood and fabric. It was a single bay biplane with wings of rectangular plan out to rounded tips and with forward stagger, braced together with N-form interplane struts. The wing centre-section was held well above the fuselage on pairs of inverted V-struts on each side and another pair of single struts from the lower fuselage longerons to the forward wing spars. The ailerons were on the upper wings. It was powered by a , Aichi-built water-cooled Lorraine 12E Courlis. The flat-sided fuselage included open, tandem cockpits for the crew and a four seat passenger cabin below the upper wing with a pair of small windows on each side, the forward one in a rear-hinged door that opened ahead of the lower leading edge. The tail was conventional, with a tailplane mounted high on the fuselage and a cropped triangular profile fin. Both rudder and elevators were balanced. The AB-1 landplane had fixed, split axle landing gear with axles parallel to the ground, their inner ends hinged on a V-strut from the lower fuselage. Both its landing legs, with prominent shock absorbers, and its rearwards drag struts were mounted on the lower fuselage longerons. One of the Aircraft Bureau's requirements was for ready exchange between landplane and seaplane undercarriages. The floats added to the overall length and increased the empty weight by 21%. The extra drag