Henschel Hs 129 The Henschel Hs 129 was a World War II ground - attack aircraft fielded by the German " Luftwaffe " . Its nickname , the " Panzerknacker " ( tank cracker ) , is a deliberate pun — in German , it also means " safe cracker " . In combat service the Hs 129 lacked a sufficient chance to prove itself ; the aircraft was produced in relatively small numbers and deployed during a time when the " Luftwaffe " was unable to protect them from attack . Design and development . By the middle of the 1930s , the idea of using aircraft against ground targets had been widely understood to be of little use other than hurting enemy morale . Experiences during World War I had demonstrated that attacking the combatants was generally much more dangerous to the aircraft than the troops on the ground , a problem that was only becoming more acute with the introduction of newer weapons . For much of the 1920s and 1930s , the use of aircraft was seen primarily in the strategic and interdiction roles , where their targets were less likely to be able to fight back with any level of coordination . For high - value point targets , the dive bomber was the preferred solution . The German Condor Legion experience during the Spanish Civil War turned this idea on its head . Though equipped with generally unsuitable designs such as the Henschel Hs 123 and cannon - armed versions of the Heinkel He 112 , their armament and pilots proved that the aircraft was a very effective weapon even without bomb s . This led to some support within the " Luftwaffe " for the creation of an aircraft dedicated to this role , and eventually a contract was tendered for a new " attack aircraft " . Since the main source of damage would be from rifle and machine gun fire from the ground , the plane had to be heavily armored around the cockpit and engines . Similar protection was also needed in the windscreen , which required 75 mm ( 2 . 95 in ) thick armored glass . The aircraft was expected to be attacking its targets directly in low - level strafing runs , so the cockpit had to be located as close as possible to the nose in order to see the ground . One last requirement , a non - technical one , ended up dooming the designs : the RLM demanded that the aircraft be powered by " unimportant " engines of low horsepower that were not being used in other designs , so the plane ' s production would not interfere with that of other types deemed more essential to the war effort . Four companies were asked to respond , and only two of the resulting three entries were considered worthy of consideration : Focke - Wulf ' s conversion of their earlier Fw 189 reconnaissance plane , and Henschel ' s all new Hs 129 . Prototypes . The Hs 129 was designed around a single large " bathtub " of steel sheeting that made up the entire nose area of the plane , completely enclosing the pilot up to head level . Even the canopy was steel , with only tiny windows on the side to see out of and two angled blocks of glass for the windscreen . In order to improve the armor ' s ability to stop bullets , the fuselage sides were angled in forming a triangular shape , resulting in almost no room to move at shoulder level . There was so little room in the cockpit that the instrument panel ended up under the nose below the windscreen where it was almost invisible ; some of the engine instruments were moved outside onto the engine nacelles , as on some models of Messerschmitt ' s Bf 110 heavy fighter , and the gunsight was mounted outside on the nose . Henschel ' s plane came in 12 % overweight with the engines 8 % underpowered , and understandably , it flew poorly . The controls proved to be almost inoperable as speed increased , and in testing , one plane flew into the ground from a short dive because the joystick forces were too high for the pilot to pull out . The Focke - Wulf design proved to be no better . Both planes were underpowered with their Argus As 410 engines , and very difficult to fly . The RLM nevertheless felt they should continue with the basic concept . The only real deciding factor between the two designs was that the Henschel was smaller and cheaper . The Focke - Wulf was put on low priority as a backup , and testing continued with the Hs 129 A - 0 . A series of improvements resulted in the Hs 129 A - 1 series , armed with two 20 mm MG 151 / 20 cannon s and two 7 . 92 mm ( . 312 in ) MG 17 machine gun s , along with the ability to carry four 50 kg ( 110 lb ) bombs under the fuselage centreline . Hs 129 B - 1 . Even before the A - 1s were delivered , the plane was redesigned with the Gnome - Rhône 14M radial engine , which were captured in some number when France fell and produced under German occupation . This engine supplied 522 kW ( 700 hp ) for takeoff compared to the Argus at 347 kW ( 465 hp ) . The Gnome - Rhone radials were also made in versions with opposite rotation for the propeller , and were installed on the Hs 129 with the port engine rotating clockwise and the starboard rotating counterclockwise — as seen from nose - on — thus eliminating engine torque problems . The A - 1 planes were converted into Hs 129 B - 0s for testing ( although it has been claimed that some As were sold to Romania ) and the pilots were reportedly much happier . Their main complaint was the view from the canopy , so a single larger windscreen and a new canopy with much better vision were added , resulting in the production model Hs 129 B - 1 . B - 1s started rolling off the lines in December 1941 , but they were delivered at a trickle . In preparation for the new plane , I . / SchlG 1 had been formed up in January with Bf 109 E / Bs ( fighter - bomber version of Bf 109 E ) and Hs 123 s , and they were delivered B - 0s and every B - 1 that was completed . Still , it wasn ' t until April that 12 B - 1s were delivered and the 4th " staffel " ( squadron ) became ready for action . They moved to the Eastern Front ( to the Crimea ) in the middle of May 1942 , and in June they received a new weapon , the 30 mm ( 1 . 2 in ) MK 101 cannon with armor - piercing ammunition in a centerline pod . Hs 129 B - 2 . Deliveries of the new Hs 129 B - 2 model began in May 1942 , side - by - side with the B - 1 ( of which just 50 planes had been delivered at that point ) . The only difference between the two were changes to the fuel system – a host of other minor changes could be found almost at random on either model . These changes accumulated in the B - 2 production line until they could eventually be told apart at a glance ; the main differences being the removal of the mast for the radio antenna , the addition of a direction - finding radio antenna loop , and shorter exhaust stacks on the engines . In the field , the differences seemed to be more pronounced . The " Rüstsatz " field refit kits were renumbered and some were dropped , and in general , the B - 2 planes received the upgraded cannon pack using a 30 mm MK 103 cannon instead of the earlier MK 101 . These guns both fired the same ammunition , but the 103 did so at almost twice the rate . Hs 129 B - 3 . By late 1942 reports were coming in about the ineffectiveness of the MK 103 against newer versions of the Soviet T - 34 tank s . One obvious solution would be to use the larger " Bordkanone " BK 3 , 7 gun , recently adapted from the ground - based Flak 18 . These guns had already been converted into underwing pod - mounted weapons for the Ju 87 and found to be a fearsome weapon . When mounted on the Hs 129 , the empty area behind the cockpit could be used for ammunition storage , which would address the only problem with the Ju 87 ' s mounting : a limited ammunition supply . Few Hs 129s were actually installed with the BK 3 , 7 however , and the Rheinmetall firm decided to adapt for the aircraft ( as had already been done with the heavy - gunned Ju 88 P - 1 ) their semi - automatic loading 7 . 5 cm Pak 40 anti - tank gun into a lighter - weight , fully automatic aircraft - mountable version , with a completely different and more aerodynamic muzzle brake to produce the " Bordkanone " BK 7 , 5 model . A huge hydraulic system was used to dampen the recoil of the gun , and an autoloader system with 12 rounds in the magazine was fitted in the large empty space behind the cockpit . This made for an easier design solution due to the PaK 40 already having a semi - automated firing mechanism . The gun and its recoil mechanism occupied a substantial gun pod under the fuselage and a hole in the rear end of the pod allowed spent cartridges to be ejected . The resulting system was able to knock out any tank in the world , but the added weight further hindered the already poor performance of the airplane . The Hs 129 B - 3 version was a very poorly handling aircraft . B - 3s finally started arriving in June 1944 , and just 25 were delivered by the time the lines were shut down in September . A small number were also converted from older B - 2 models . In the field they proved deadly weapons , but with only 25 aircraft available they had no effect on the war effort . The only other aircraft to serve in World War II that were factory - equipped with as heavy a calibre of cannon as the B - 3 version possessed were the 1 , 420 examples of the North American B - 25G and B - 25H Mitchell attack bombers , internally mounting the lower muzzle velocity M4 cannon or lighter weight T13E1 and M5 aircraft versions of the same cannon , all hand - loaded , mounted entirely within the fuselage , and of shorter barrel length than the BK 7 , 5 . The 1 , 200 kg ( 2 , 645 lb ) " Bordkanone " BK 7 , 5 cannon installation in the Hs 129B - 3 was the heaviest forward - firing " big - gun " installation 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 ammunition included . Hs 129 C . In order to address the poor performance of the aircraft , plans had been underway for some time to fit the plane with newer versions of the Italian Isotta - Fraschini Delta engine that delivered 630 kW ( 850 hp ) . The engine installation ran into a number of delays however , and was still not ready for production when the plant was overrun by the Allies in 1945 .