Luftwaffe using Junkers Ju-87 'Stuka' dive-bombers effectively acting as artillery. Conventional towed howitzers followed. As the war progressed, most nations developed self-propelled artillery. Some early attempts were often no more than a field gun or anti-tank gun mounted on a truck—a technique known in the British Army as carrying portee. These were mobile, but lacked protection for the crew. The next step was to mount the guns on a tracked chassis (often that of an obsolete or superseded tank) and provide an armoured superstructure to protect the gun and its crew. Many of the early designs were improvised and the lessons learned led to better designs later in the war. For example, the first British design, "Bishop", carried the 25 pdr gun-howitzer on a Valentine tank chassis, but in a mounting that severely limited the gun's performance. It was replaced by the more effective Sexton. The first battery of self-propelled artillery guns was created when Hauptmann Alfred Becker, a mechanical engineer and battery captain in the 227th Infantry Division, mounted his 10.5 cm leFH 16 howitzers on the chassis of captured British Vickers Mk.VI light tanks to mobilize his guns. His 10.5 cm leFH 16 Geschützwagen Mk VI 736 (e) was the forerunner to German tracked field guns such as the Wespe and Hummel. The Germans also mobilized their anti-tank guns, using light, obsolete or captured tracked vehicles. Examples include the Marder I, using captured French Lorraine 37L tractors, the Marder II, using the Panzer II light tank chassis, and the Marder III, which was based on the Panzer 38(t) Czech chassis. These led to better-protected Assault guns – Sturmgeschütz – with fully enclosed casemates, built on medium tank chassis. In the same way self-propelled anti-tank guns such as the Jagdpanzer IV and Jagdpanther. Some designs were based on existing chassis (such as the Brummbär), leftover chassis from cancelled programs (Elefant and Sturer Emil); others were converted from battle-damaged tanks (Sturmtiger). The single most-produced armored fighting vehicle design for Germany in WW II, the Sturmgeschütz III (StuG III) assault gun, in 1936-37 pioneered the later casemate-style fully enclosed armor that would be used on almost all late-war German self-propelled artillery and Jagdpanzer-format tank destroyers. The Soviets experimented with truck-and tank-based self-propelled weapons, producing a few Komsomolets tractor-mounted 57-mm ZiS-2 guns early in the war. By 1943, the series of Samokhodnaya Ustanovka casemate-armored vehicles had started to appear at the front,