Schräge Musik, derived from the German colloquialism for "Jazz Music" (the German word "schräg" literally means "slanted" or "oblique"; it also has a secondary meaning of "weird", "strange", "off-key" or "abnormal" as in the English "queer"), was the name given to installations of upward-firing cannon mounted in night fighters by the Luftwaffe and Japanese Imperial Navy during World War II. This allowed them to approach and attack British bombers from below, where they would be outside the bomber crew's field of view. Few bombers of that era carried defensive guns in the ventral position. The ventral turret fitted to some early Lancasters was sighted by periscope from within the fuselage, and proved of little use—the fitting of a Sperry ball turret of the kind fitted to the B-17 Flying Fortress and B-24 Liberator would not have been practical as the British bombers had not been designed to use them—and sighting a target at night, for the ball turret gunner to aim at, might have been a problem as well.
During World War I, forward firing Lewis Guns were frequently mounted on the top plane of a biplane to fire over the revolving propeller, due to the difficulty of synchronising this type of weapon to fire through the propeller arc. Since the gun had to be tilted back to change ammunition drums it followed that the gun could also be fired upwards at an angle. The guns of the Nieuport 11 and Nieuport 17 fighters, especially in British service, and the Royal Aircraft Factory S.E.5a were often used in this way, to attack enemy bomber or reconnaissance aircraft from the "blind spot" below the tail. The British ace Albert Ball, in particular, was a great exponent of this technique. The first "multi-gun" fighter design to see combat, the Sopwith Dolphin, featured an "as-equipped" armament setup of two forward-firing Vickers machine guns in the usual location just forward of the cockpit, but also had a pair of Lewis machine guns located on the forward cross-tube that comprised part of the cabane strut structure, and intended to be aimed forwards and upwards as an anti-Zeppelin armament scheme. The concept of off-set or oblique mounted weaponry in night fighting was first used by Royal Aircraft Factory B.E.2c night fighters to attack airships from below—this was highly successful, and most British night fighters for the remainder of the war (for instance the Sopwith Camels of No. 151 Squadron RAF in 1918) used wing mounted Lewis guns in preference to the synchronised Vickers preferred for day fighting—in part for this reason. Similar arrangements were trialled by the Germans in 1917, when Gerhard Fieseler of Jasta 38 fitted two machine guns pointing upwards and forwards.
The Westland C.O.W. Gun Fighter had been another example of a trial of upward tilting guns, in this case the Coventry Ordnance Works 37 mm autocannon in the 1930s. The Boulton Paul Defiant and Blackburn Roc were two British fighter aircraft that carried their armament in turrets giving them a wide range of fire including upwards. The Defiant was put into service in 1939 with the intention to be used against bombers despite the bombers' numerous gun positions. After becoming outclassed as a day fighter, the Defiant moved to the night fighter role where it had some success at night—typically attacking from below and slightly ahead of the target bomber, and well out of its field of defensive fire.
Prior to the introduction of Schräge Musik in 1943, German night fighters were simply heavy fighters equipped with radar in the nose. This meant that the fighter had to approach the target bomber from the rear in order to get into a firing position. This presents a much smaller target area and Royal Air Force bombers were all equipped with multi-gun hydraulic rear turrets to help fend off such attacks.
Night fighter pilots then developed a new tactic to avoid the turrets. Instead of approaching directly from the rear, they would approach about 1500 ft below the plane. They would then pull up sharply and start firing when the nose of the bomber appeared in the gunsight. As their plane slowed and the bomber passed over them, the wings were sprayed with cannon or machine gun rounds. This maneuver was effective, but difficult to perform. There was a significant risk of collision and if the bomb-load exploded, it could take down the night fighter too.
With Japanese Imperial Navy, in 1943, Commander Yasuna Kozono of the 251st Kokutai in Rabaul came up with the idea of converting the Nakajima J1N (J1N1-C) into a night fighter. The field-modified J1N1-C KAI shot down two B-17's of 43rd Bomb Group attacking air bases around Rabaul on 21 May 1943. The Navy took immediate notice and placed orders with Nakajima for the newly designated J1N1-S nightfighter design. This model was christened the Model 11 Gekko (月光, "Moonlight"). It required only two crew and like the KAI, had a twin 20 mm pair of Type 99 Model 1 cannon firing upward and a second pair firing downward at a forward 30° angle, placed in the fuselage behind the cabin.
Oberleutnant Rudolf Schoenert of 4./NJG 2 decided to experiment with upward firing guns in 1941 and began trying out upward-firing installations amidst scepticism from his superiors and fellow pilots. The first installation was made late in 1942, in a Dornier Do 17Z-10 that was also equipped with the early UHF-band version of the FuG 202 Lichtenstein BC radar. Further experimentation was carried out by the Luftwaffe weapons testing centre at Tarnewitz through 1942, and an angle of between 60 and 75 degrees was found to give best results.
Meanwhile, Schönert was made CO of II./NJG 5, and an armourer serving with the Gruppe, Oberfeldwebel Mahle developed a working arrangement with the unit's Bf-110's and a pair of MG FF/M 20 mm cannon. Schönert used such a modified Bf-110 to shoot down a bomber in May 1943. From June 1943 on, an official conversion kit was produced for the Junkers Ju 88 and Dornier Do 217N fighters.
Wide-scale adoption followed in late 1943, and in 1944 a third of all German night fighters carried upward-firing guns. The Revi 16N gunsight was modified to allow the pilot to aim at the target by placing a reflecting mirror above the pilot’s head, while the sight itself was further to the rear. An increasing number of these installations used the more powerful 30 mm calibre, short-barreled MK 108 cannon, such as those fitted to the Heinkel He 219. The installation contributed significantly to the successes of the German nightfighter force in the winter of 1943-1944. The definitive night fighter version of the Messerschmitt Me 262, the Me 262B-2, was also designed to carry such an installation, but none were built before the end of the war.
Schräge Musik proved to be most successful on the Jumo 213 powered Ju 88G-6, which was both fast and manouevreable. Using the Schräge Musik (or Schrägwaffen, as it was also called) required precise timing and swift evasion; a fatally damaged bomber could fall directly upon the night fighter who had just shot it down if the fighter could not quickly turn away. The He 219 was particularly prone to this; its high wing loading left it unmanouevreable, and the 61-victory night fighter ace Manfred Meurer lost his life 21/22 January 1944 as a Handley Page Halifax bomber he had just shot down fell upon his He 219.
The Japanese Army Air Force's Mitsubishi Ki-46 "Dinah" twin engined fighter was used to test the Schräge Musik armament format in its Ki-46 III KAI version in June 1943, using a 37 mm Ho-203 cannon with 200 rounds of ammunition. It was mounted in virtually the same position in the fuselage as in the Luftwaffe fighters. Operational deployment began in October 1944. One of the main Japanese fighters using this device was the Kawasaki Ki-45 "Nick". The Japanese Navy air forces also used the Schräge Musik installation with the Nakajima J1N1-S "Gekko" (two or three 20mm cannons firing upwards, some had two firing downwards), and a pair of 20 mm Type 99 cannons in the Nakajima C6N1-S "Myrt".
The American purpose-built Northrop P-61 Black Widow night fighter could, because of the design of its remote dorsal turret carrying a quartet of the powerful Browning M2 machine guns, deliver a Schräge Musik–like surprise of its own from below an enemy aircraft if desired in combat. The Black Widow was the only night fighter of either side that had such a fully-traversable dorsal turret, as part of its design.
Similar systems to the original Schräge Musik cannon fitment format were tested on day fighters as well, such as the Sondergeräte series of weapon systems (on a variety of airframes, including the Heinkel He 177), and the Messerschmitt Me 163's Jägerfaust.
An attack by a Schräge Musik equipped fighter was typically a complete surprise to the bomber crew, who would only realise that a fighter was close by when they came under fire. Particularly in the early stage until spring 1944, the sudden fire from below was often attributed to ground fire rather than a fighter.
Freeman Dyson, who was an analyst for Operations research of RAF Bomber Command in World War II, commented on the effectiveness of Schräge Musik: "The cause of losses ... killed novice and expert crews impartially. This result contradicted the official dogma...I blame the ORS and I blame myself in particular, for not taking this result seriously enough...If we had taken the evidence more seriously, we might have discovered Schräge Musik in time to respond with effective countermeasures."