During 1936 the Imperial Japanese Navy issued an specification for a monoplane dive bomber that could replace the biplane Aichi D1A. Aichi, Nakajima and Mitsubishi presented their designs and only Aichi and Nakajima were granted with prototypes building permissions.
The D3A was inspired by the Heinkel He.70 and it's fixed landing gear wasn't a problem when flying at low speed because the drag was unappreciable so, in order to make the design simpler and easier to manufacture, it wasn't changed. It was expected to be powered by the 709hp Nakajima-Hikari 1 radial engine.
In December 1937 the first prototype was completed and in January 1938 it flew for the first time, however the first tests were disappointing: it was underpowered and was very unstable, specially in wide and tighter turns that would normally lead to a snap roll. The dive brakes were also a problem as they added excesive vibration and slowed down the dive too much.
In order to fix all those problems, a second prototype was built. It was fitted with a 839hp Mitsubishi Kinsei 3, the cowling was redesigned and the vertical tail was enlarged to fix the maneouvrability problems. Wings were also enlarged and the tips were washed-out to cope with the snap rolls. Dive brakes were also replaced with new, more strengthned ones. All those changes fixed almost of it's problems except for the directional stability, but it was, however, enough to make it win against it's most direct rival, the Nakajima D3N1.
Sources:
1. https://en.wikipedia.org/wiki/Aichi_D3A
2. http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=296
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