The Biber (German for "beaver") was a German midget submarine of the Second World War. Armed with two externally mounted 53-centimetre (21 in) torpedoes or mines, they were intended to attack coastal shipping. They were the smallest submarines in the Kriegsmarine.The Biber was hastily developed to help meet the threat of an Allied invasion of Europe. This resulted in basic technical flaws that, combined with the inadequate training of their operators, meant they never posed a real threat to Allied shipping, despite 324 submarines being delivered. One of the class's few successes was the sinking of the cargo ship Alan A. Dale.
The hull was built in three sections composed of 3 millimetres thick steel with an aluminium alloy conning tower bolted to the top. The conning tower contained armoured glass windows to allow the pilot to see out. The hydroplanes and rudder were made of wood and trying to control them while tracking the depth gauge, compass and periscope made the craft hard to handle. Adding to the pilot’s difficulties, the craft lacked compensating and trimming tanks, making staying at periscope depth a near impossibility. The Biber had two diving tanks, one in the bow section and one in the stern. The submarine could be armed with either two TIIIc torpedoes with neutral buoyancy, mines, or a mixture of the two. The torpedoes or mines were accommodated in semi-circular recesses in the side of the hull. These reduced the overall width of the loaded craft, making land transport easier and also reduced drag in the water, but at the cost of weakening the hull. The Biber was powered on the surface by a 32 hp Otto Blitz petrol engine, which was used despite concerns about the risks posed by the carbon monoxide the engine gave off. The engine had the advantage of being cheap and available in large numbers. Propulsion while submerged was provided by a 13 horsepower electric motor, supplied by three Type T13 T210 battery troughs.