Background By the mid-1930s, the German military, as well as its counterparts in other countries, had identified the importance of ground-attack aircraft for the
interdiction of logistics and
materiel, a task in which targets were often poorly protected and less likely to have strong or well-coordinated defences. For high-value, well-protected tactical targets, the
dive bomber was becoming the conventional solution. The experience of the German
Condor Legion during the
Spanish Civil War (1936–39) refuted this concept. Even though it was equipped with types unsuited to the role, such as the
Henschel Hs 123 and
cannon-armed versions of the
Heinkel He 112, the
Condor Legion proved that ground-attack aircraft were effective in actual combat. This determination led to support within the
Luftwaffe for the creation of an aircraft dedicated to this role, and the
Reichsluftfahrtministerium (RLM; "
Reich Aviation Ministry") through its
Technisches Amt (Technical office) formulated a specification for a
Schlachtflugzeug (ground-attack aircraft), specifically intended for the direct support of ground forces. Of these, Gotha opted to not respond at all while the Hamburger Flugzeugbau offered their
P-40 project, a development of their two seat
Ha 141, however, the ground attack version was considered to be too unconventional and was eliminated. On 1 October 1937, the RLM issued a pair of development contracts for the two other submissions, which had been determined to superior: one was derived from an existing Focke-Wulf reconnaissance type, the
Fw 189, while the other was produced by Henschel.
Prototypes During 29 May 1939, the first prototype, designated
Hs 129 V1, performed its
maiden flight. Henschel's aircraft came in 12 percent overweight while the engines produced eight percent less power than had been anticipated; accordingly, it flew poorly. A further complication was the poor external visibility available to the pilot from the cramped cockpit. The Focke-Wulf design proved to be no better. Both aircraft were underpowered as a result of their air-cooled, inverted-V12
Argus As 410 engines as well as being relatively difficult to fly. Nevertheless, the RLM felt they should continue with the concept. The only real deciding factor between the two designs was that the Henschel was smaller and cheaper, thus a further development contract was issued to Henschel for eight
HS 129 A-0 pre-production aircraft while the FW 189 was positioned on low priority as a backup while testing continued with the Hs 129 A-0.
Hs 129 B-1 Even prior to the delivery of the first A-1s, the aircraft already being redesigned in response to an instruction from the RLM to adopt French
Gnome-Rhône 14M radial engines, which were captured in some number when
France fell and continued to be produced under German occupation. This engine supplied for takeoff, compared with the Argus at . 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. During early 1941, a pair of HS 129 A-1s were refitted with the Gnome-Rhône radials and quickly demonstrated that they possessed more satisfactory performance. Although reviewing pilots were more satisfied with the aircraft's performance, complaints continued to be filed about the high stick forces as well as the poor visibility from the canopy. In preparation for the arrival of the B-1, I./
SchlG 1 had been formed in January 1942 with
Bf 109 E/Bs (fighter-bomber version of Bf 109 E) and Hs 123s and they delivered B-0s and every B-1 that was completed. During April 1942, the first 12 B-1s were delivered, enabling the 4th
Staffel (squadron) to achieve operational readiness. They were promptly assigned to the
Eastern Front (to
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. This would be the B1/R2
Rüstsätze kit, the B1/R3 had the MK 101 replaced with a ventral tray of four 7.92mm MG 17 machine guns. The B1/R4 had a larger bomb load and the B1/R5 reconnaissance version had a camera.
Hs 129 B-2 During May 1942, deliveries of the new
Hs 129 B-2 model commenced, side by side with the B-1 (of which just 50 aircraft had been delivered at that point). By July 1943, (prior to the launch of
Operation Citadel, these were a total of five units of the
Luftwaffe equipped with HS 129s. Initially, the only difference between the B-1 and B-2 models 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 became more pronounced. The
Rüstsatz field refit kits were renumbered and some were dropped, and in general, the Hs 129B-2/R2 aircraft in service with 4./Schlachtgeschwader 1 received the upgraded cannon pack using a 30 mm
MK 103 cannon instead of the earlier MK 101. The MK 103 was effectively an upgraded MK 101 that was capable of increased muzzle velocity. By late 1942, reports were coming in about the ineffectiveness of the MK 101 against newer versions of the
Soviet T-34 tanks. One obvious solution would be to use the larger
BK 3,7 (
Bordkanone 3,7), recently adapted from the ground-based
3.7 cm Flak 18. These guns had already been converted into underwing pod-mounted weapons for the
Junkers Ju 87G and found to be an effective weapon, despite the fact that only 12 shells per pod could be accommodated. When mounted on the Hs 129, the empty area behind the cockpit could be used for ammunition storage, greatly increasing the supply compared to the Ju. The B-2/R3 package introduced the BK automatic cannon, but relatively few aircraft were converted in favour of the B-2/R4 and B-3, mounting the BK 7.5.
Hs 129 B-3 It was decided that the semi-automatic
Rheinmetall PaK 40 anti-tank gun, which had already been adapted for use in the
Junkers Ju 88 P-1, would be further modified for use in the Hs 129. The
Bordkanone 7,5 was the heaviest and most powerful forward-firing weapon fitted to a production
military aircraft during the Second World War. The only other ground-attack 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 lightweight T13E1 or M5 versions of the same gun, both of which required a crew member to manually reload after each shot. The Japanese
Mitsubishi Ki-109 Army Heavy Fighter Interceptor also carried a 75mm gun, but only 24 were produced. In May 1944, a Hs 129B-2,
Werknummer 141258, was fitted with a mock-up of the BK 7.5 and underwent aerodynamic testing at
Travemünde. The gun fired a 12 kg shell and in an emergency the entire installation could be jettisoned. During August of that year, the first three HS 129 B-3s commenced trials.
Hs 129 D Planned version of the Hs 129. Powered by either two
Junkers Jumo 211 or two
BMW 801 to improve its performance. No prototypes were made. ==Operators==