The 7.5 cm BK (Bordkanone 7.5 cm) was a anti-tank cannon designed and produced by Rheinmetall-Borsig in 1943. The 7.5 cm BK was basically a Luftwaffe modified 7.5 cm PaK 40 designed to be mounted in a ventral gondola pod beneath the aircraft fuselage and in an emergency could be jettisoned, this weapon, a 12 round magazine.

A lighter automatic "weapon system" version incorporating a twelve-round magazine, the heaviest of the Bordkanone series of heavy calibre aircraft guns. It was used as the BK 7,5 in the Henschel Hs 129B-3 ground attack aircraft and the Junkers Ju 88P-1 bomber destroyer, and even intended as a production fitment for a possible He 177A-3/R5 heavy bomber adaptation late in 1942. 

Henschel Hs 129B-3 closeup. This aircraft/weapon combination no longer exists. A close up of the Bordkanone BK 7,5 75 mm cannon [Via]

Ju 88P-1: Heavy-gun variant fitted with single 75 mm (2.95 in) Bordkanone BK 7,5 cannon in ventral gun pod. Appeared in mid-1942 in small numbers, they were perceived as a failure for both anti-tank  and anti-bomber use [Via/Via]

The only other aircraft to be factory-equipped with similar-calibre guns were the 1,420 examples of the North American B-25G and B-25H Mitchell, which mounted either a M4 cannon, or light-weight T13E1 or M5 versions of the same gun. These weapons, however were hand-loaded, had shorter barrels and/or a lower muzzle velocity than the BK 7,5, resulting in lesser ballistic performance, accuracy and rate of fire.

B-25H 'Barbie III' showing 75 mm M5 gun and 4 Browning 50 cal feeds. A lightweight version of the M3 with a lighter thin-walled barrel and a different recoil mechanism that was used in the B-25H Mitchell bomber. Uses the same ammunition and has the same ballistics as the M3 [Via]

The BK 7,5 was unsurpassed as a production aircraft-fitted gun until 1971, when the four-engine Lockheed AC-130E Spectre – equipped with a sideways-aimed, hand-loaded 105 mm M102 howitzer of about 3,298 lb (1,496 kg) apiece – entered service with the U.S. Air Force.

The 1,200 kg (2,645 lb) Bordkanone BK 7,5 cannon installation in the Hs 129B-3 was the heaviest forward-firing autocannon ever made for a series production military aircraft, until the introduction of the Fairchild Republic A-10 "Warthog", with its General Electric GAU-8 Avenger seven barrel 30mm caliber anti-tank Gatling cannon main armament coming in at a total weight of up to 1,830 kg (4,030 lb) with an ammunition load of some 1,100 shells included in a drum magazine integral to the weapon system, much like the much smaller 12-round magazine of the BK 7,5.