In 1937 the Technische Amt issued specifications for a close-support, with a cannon, aircraft. The competing designs were the Focke-Wulf 189 and the Henschel Hs 129. Both didn't perform well and the Hs 129 won mostly due to the fact it was 2/3s the cost of the Fw 189. Friedrich Nicolaus was the designer of the Henschel Hs 129.
The Henschel Hs 129 was designed from the start to destroy tanks.
Some of the Hs 129B-2s had six 75 mm 5' 3" / 1.6 m long tubes mounted at an angle in the fuselage. They were fired automatically when it flew over a tank at low altitudes.
The Hs 129 was designed to have a small cross section and this resulted in the cockpit being very small and cramped. Some pilots couldn't fit into the small cockpit.
The cockpit had 0.5" / 12 mm armored plating on the floor and the front. There was 0.25" / 6 mm armor on the sides.
The gun sight was mounted in front of the canopy.
The engine controls were mounted on the engine nacelles.
The undercarriage retracted rearwards into the engine nacelles with a little bit of the wheel left exposed to help minimize damage in the case of a wheels up landing.
The fuselage was light metal with a stressed skin. This was joined to the armored cockpit.
The bottom was armored to protect against ground fire.
The Hs 129 would have been a great close support aircraft except for its engine problems. The initial Argus engine was underpowered, and the Gnome-Rhône were vulnerable to ground fire and prone to fail.