The J Wal features metal construction with fabric-covered wings and control surfaces, a hull design that incorporates distinctive, patented sponsons for stability on water, and a traditional empennage. The main wing, which has a span of 72 feet, 2 inches, is elevated above the hull by a lattice structure and is reinforced by struts connected to the sponsons. The J Wal is powered by two engines in a tractor-pusher configuration mounted in a nacelle above the main wing. J Wals use various engines in pairs, including the Rolls-Royce Eagle IX V-12 that produces up to 355 horsepower, the 500-horsepower Napier Lion V, and the BMW V12x that turns out up to 600hp, among others.
Depending on engines and aircraft configuration, the J Wal cruises between 87 and 106 mph, has a top speed of 124 to 130 miles per hour, and has a range from 497 to 621 miles. It climbs at 300 feet per minute and has a service ceiling of between 10,170 and 14,764 feet above sea level.