In 1936 the Imperial Japanese Navy issued specifications for a dive-bomber to replace the Aichi D1A2 biplanes. Aichi, Mitsubishi, and Nakajima submitted designs. Aichi and Nakajima were asked to construct prototypes.
Aichi delivered a design that had a fixed undercarriage and a wing that was inspired by the German He 70. It was decided that the complexity and extra weight of a retractable landing gear wasn't worth the small increase in performance.
Initial tests showed a number of problems that resulted in a redesign. This aircraft was then chosen over the Nakajima D3N1. There were still directional stability problems that resulted in a dorsal fin being installed. This was done before carrier qualification testing that occurred in 1940.
The D3A was constructed of metal.
The wings were placed low in the fuselage.
The D3A used a fixed faired undercarriage.