Saab’s design was for a shoulder-fired recoilless rifle capable of firing 84 mm anti-tank ammunition. Recoilless rifles operate on the principle that equal but opposite forces counteract each other.

The ’Davis Gun’ was not successful in putting big guns on biplanes. But the design evolved into a series of large caliber U.S. Army anti-tank weapons known as Recoilless Rifles mounted in ...