What material were the fuel tanks of the Beech AT-10 trainer made of?

Beechcraft began designing the Model 25 in early 1940 in response to a requirement from what was then called the United States Army Air Corps (USAAC) for a small twin-engine aircraft suitable for training students in handling multi-engine aircraft with retractable landing gear. Since there was concern at the time about a possible future shortage of aluminum, part of the requirement was that the aircraft be built using "non-strategic" materials. Beechcraft met that condition by designing the aircraft to be built mainly of wood. To conserve the scarce metals needed by combat aircraft, the fuselage was made of plywood, and only the engine cowlings and cockpit enclosure remained aluminum. The fuel tanks were also made of wood, coated with neoprene, a synthetic rubber.

The Model 25 prototype was delivered to the USAAC for evaluation, but it was destroyed in an accident on May 5, 1941. The following day Beechcraft began work on the Model 26, which was soon ready and made its first flight on July 19 of that same year. By the end of 1942, 748 aircraft had been delivered, taking part in training crews for the huge fleets of bombers and transports.