Vickers Type 161 The Vickers Type 161 was an unusual 1930s pusher biplane interceptor , designed to attack aircraft from below with a single upward - angle large calibre gun . The aircraft flew well but the concept was abandoned ; only one was built . Development . The Vickers 161 was designed in response to Air Ministry specification F . 29 / 27 . This called for an interceptor fighter operating as a stable gun platform for the COW 37 mm gun produced by the Coventry Ordnance Works ( COW ) that that fired 23 oz ( 0 . 65 kg ) shells . The gun was to be mounted at 45 degrees or more above the horizontal , so that the aircraft could fly below the target bomber or airship , and fire upwards into it . During World War II the Luftwaffe successfully used a rather similar approach , named " Schräge Musik " . The specification also called for a top speed well in excess of a typical bomber ' s cruising speed and a good rate of climb . Vickers ' approach seems to have been influenced by their World War I experience with the Gunbus family . Like them , the Type 161 was a single engined pusher biplane . The wings were of unequal span and parallel chord , mounted with heavy stagger and a large gap braced in two bay fashion by streamlined I form , outward leaning interplane struts . Parallel booms , formed on each side by a pair of tubular members converging from the top and bottom of the inner interplane struts onto the tail . Another pair of tubes joined the bottom of the interplane struts to the upper boom at midpoint . The tailplane had a wide span , extending past the booms ; the fin and rudder were conventional and stiffened with lighter bracing to mid - boom . Flying surfaces were fabric covered . Pilot and gun were housed in a metal monocoque nacelle mounted to the underside of the upper wing , leaving a gap below . The pilot ' s cockpit was off - set to port with the gun to his right , its breech accessible . The Bristol Jupiter VIIF was installed with its cylinders in line with the rear edge of the upper wing , supported by two pairs of struts to the lower wing spars and driving a four blade propeller . This had an unusual ring fairing that rotated with it and matched the engine cowling in diameter . Aft , and without a break , a fuselage - like fairing ran rearwards , narrowing to the tail . This structure was stabilized on each side by a pair of struts to the upper and lower booms . A split axle undercarriage had legs to the fuselage and , rearwards , to the forward wing spar , with a strut between their upper joints . The Type 161 flew for the first time on 21 January 1931 . Further flight trials produced some modifications , largely to improve yaw stability . The rudder was broadened and rounded at the top , and small fins were added above and below the tailplane at the boom mounting point . There were also alterations to the geometry and gearing of the elevator trim tabs : it has been suggested that the Type 161 may have been the first aircraft to have had in - flight adjustable elevator trims . In September 1931 it went to RAF Martlesham Heath for trials , where no serious problems emerged and pilot ' s reports were positive . The gun firing tests went well , with no detriment to airframe or performance . Despite that , neither the Type 161 or its competitor the Westland C . O . W . Gun Fighter were ordered and no more was heard of the aerial COW gun .