The design for the Yokosuka D4Y was based on the purchased design of the German He 118 (DXHe1).
When the original He 118V4 was destroyed, the Imperial Japanese Navy as the Dai-Ichi Kaigun Koku Gijitsusho (First Technical Arsenal of Naval Aviation) in Yokosuka to design a replacement following the design of the He 118.
The requirements of the D4Y were:
- Cruising Speed: 265 mph
- Speed: 322 mph
- Range with 551 lb / 250 kg bombs: 921 miles
- Range with no bombs: 1,381 miles
- Operate from small carriers
Chief Engineer Masao Yamana headed the design team. The D4Y design was smaller and lighter than the He 118.
The D4Y was supposed to replace the Aichi D3A but it wasn't well armed and the engines were unreliable.
The D4Y carried its bomb load internally.
The construction of the D4Y was all metal.
There was no armor protection for the crew or fuel tanks.
The engine was based on an imported Daimler-Benz DB 600G engine. Initially there were problems with the Aichi Atsuta model, the DB 601A, and this delayed the D4Y.
The D4Y was one of two mass produced aircraft that used liquid cooled engines in the Japanese military, the Ki-61 Hien being the other.
The engines were the D4Y's Achilles heel.
The wing span was made short enough to not need to be folded on carriers.
There were three electric powered dive brakes in each wing.