In every modern army, all infantry is motorized. In other words, it is transported (both on, to and from the battlefield) in a specially designed motor vehicles – armored personnel carriers (APC).
And the first mass-produced and operationally deployed (in huge numbers – over 15,000 were produced) armored personnel carrier was the famous half-track Sonderkraftfahrzeug 251 (“Special Purpose Vehicle” 251) better known by the acronym Sd.Kfz. 251 or by the name of the company that designed it – Hanomag.
Interestingly enough, the first ever APC (actually, ersatz-APC) was fielded by the German Army as well – in World War I. The first mechanized infantry in the military history were assault teams mounted on German A7V tanks. The vehicles were extra-large to let them carry sizeable assault teams and would regularly carry infantry on board in addition to their already large crews that were trained as storm troopers.
All machine-gun-armed A7V tanks carried two small flame throwers for their dismounts to use. A7V tank would often carry a second officer to lead the assault team.
Development and deployment of Sd.Kfz. 251 was the inevitable result of the adoption of blitzkrieg strategy by the Wehrmacht in mid-1930’s. This strategy required lightning-fast, massive attacks by tanks and assault guns supported by infantry that had to (1) keep up with tanks on just about any terrain and (2) be protected from rifle and light machine gun fire and from mortar and artillery shell fragments.
Which could be accomplished only by mounting infantry on armored personnel carriers thus creating motorized infantry (called Panzergrenadier units in Wehrmacht and Waffen-SS).
Thus Sd.Kfz. 251 was designed to transport a single squad of ten Panzergrenadiers to the battlefield protected from the abovementioned enemy fire. In addition, the standard mounting of at least one (often two) MG 34 or MG 42 machine gun allowed the vehicle to provide suppressive fire for the rifle squad both while they dismounted and in combat.
Like all APC fielded in World War II, Sd.Kfz. 251 had an open-top design which provided greater situational awareness and faster egress by the infantry, as well as the ability to throw grenades and fire over the top of the fighting compartment as necessary while remaining under good horizontal cover.
The obvious downside was a major vulnerability to all types of plunging fire; this included indirect fire from mortars and field artillery, as well as small arms fire from higher elevated positions, lobbed hand grenades, even Molotov cocktails, and strafing by enemy aircraft.
There were four main model modifications (A through D), which formed the basis for at least 22 variants, which included a multiple 280- or 320-mm rocket launcher; 81 mm mortar carrier; communications vehicle, fitted with extra radio equipment for command; gun-towing tractor; assault engineer vehicle; armored ambulance; assault gun (equipped with a 75mm low velocity cannon or a 37mm cannon); flamethrower carrier; anti-aircraft vehicle equipped with a 20mm AA gun; tank destroyer equipped with a highly-effective 75mm PaK 40 anti-tank gun, etc.
However, because the German economy could not produce adequate numbers of its Sd.Kfz. 251, barely a quarter or a third of the infantry in Panzer or Panzergrenadier divisions were mechanized, except in a few favored formations. The rest were moved by truck (and on the battlefields had to mount tanks – just like their Soviet counterparts who had virtually no APCs at all).
The only army whose infantry was fully equipped with APCs was the US Army that used M2 and M3 half-trucks similar in design to Sd.Kfz. 251.