During the Second World War the company was one of many engineering firms that diversified into armaments. After the War, Hudswell Clarke was closely involved in various secret programmes, including the British nuclear weapon programme. The airframe for the first British nuclear bomb, Blue Danube was manufactured by Hudswell Clarke at its Roundhay Road plant in Leeds. The Blue Danube was 24 ft long x 62 inches diameter. It was known to the RAF as "Bomb, Aircraft, HE 10,000 lb MC". Released from 45,000 ft at 500 knots (930 km/h) its maximum velocity was 2480 ft/s (Mach 2.2). It bears a likeness to the Tallboy and Grand Slam "earthquake" bombs designed by Barnes Wallis. Wallis was a consultant on the design of Blue Danube.