Visual effects by Byron Haskin and Hans F (Fred) Koenekamp.
Some of the torpedo shots for this film got used on many a black and white Warner Brothers film over the years.
The model was used again in Destination Tokyo 1943 which itself became a source for submarine miniature stock shots for many years and in the same year Action in the North Atlantic 1943.
Unfortunately the only copy I could locate is a very murky digital copy from a VHS TV recording.
The photos below are showing the preparation of the model and I am informed were taken by Schuyler Crail who was the staff photographer for Warner Bros. at the time. The first photo I got on ebay but it was attributed to Submarine Patrol, a John Ford film from 1938. It has since been confirmed to me that it was in fact Submarine D1. The second and third photo below and the model resting on its cradle above are from the collection of Rob Barker. The second photo below was also up for auction at the same time and I was disappointingly outbid at the last second.
Below you can see the quite sophisticated internals, the large electric motors for driving the propellers as well as what looks like some form of central ballast tank with a multitude of plumbing. The model also had opening torpedo doors with a torpedo launching system as well as moving vanes for directional control. According to Rob Barker the model was built to 1/8 scale which is 1 1/2 inches on the model equals a foot on the real thing.