14TP

The 14TP was a Polish prototype for a medium cruiser tank that was never completed due to the start of the World War II.
It was designed in 1938 and was heavily based on the previous 10TP with the idea of improving the performance of this last one.
The armour on each side wouldn't be thicker than 35mm and it could have two type of turrets, either one with a 37mm Bofors cannon or one with a 47mm gun and it was planned to be powered by a 300hp Maybach engine to make it more powerful and fast.
The tank was 60% completed by the end of 1938 and the first tests were planned for March 1939, however it turned out that Maybach company (the builder of the engines) had sent two defective copies of the engines, so the tank couldn't be assembled. This leaded to the talks about the replacement of the defective parts with better ones and that, leaded to the halt of the project in the summer of 1939 and, shortly after, Germany invaded and the plans fell into German hands.

Sources:
1. https://en.wikipedia.org/wiki/14TP
2. Salamander Books - The Illustrated Encyclopedia of the World's Tanks and AFVs