The "R.31" class of British rigid airships was constructed in the closing months of World War I and comprised two aircraft, His Majesty's Airship "R.31" and "R.32". They were designed by the Royal Corps of Naval Constructors – with assistance from a Herr Müller who had defected to Britain and previously worked for the Schütte-Lanz airship company – and built by Short Brothers at the Cardington airship sheds. The airship frame was made from spruce plywood laminated into girder sections, weatherproofed with varnish, and also fireproofed. These enclosed 21 gas bags. "R.31" was the largest British airship to fly before the end of the war, and the class remains the largest mobile wooden structures ever built.