As the fronts of World War 2 expanded for Germany, there was a growing need to supply its army with viable Self-Propelled Artillery (SPA) systems for ranged, in-direct fire. These elements would operate behind the main fighting force and pummel enemy positions through large-caliber projectiles. Many such vehicles were born out of existing chassis systems by simply a field howitzer being added to the mix and a crude superstructure erected for protection.
In 1942, the need was still present and authorization was given to concerns to come up with a vehicle to suit the requirement. After several experiments and revisions involving both the Panzer III and Panzer IV tank chassis coupled to various field howitzers, it was decided that a combination of the Panzer III and Panzer IV tanks should be outfitted with the 15cm sFH 18 L/30 series gun. The drive and steering of the Panzer III series was mated to the suspension and engine fit of the Panzer IV. To this was added a superstructure, mounting hardware and howitzer to create the Panzerfeldhaubitze 18M auf Geschutzwagen III/IV (Sf) Hummel, SdKfz 165, better known as the "Hummel" ("Bumblebee").
Rheinmetall-Borsig and Alkett teamed up for the work and only the superstructure was of an all-new design - the rest of the vehicle consisted of proven, existing components. The gun platform's position over the rear of the vehicle required that the engine be relocated to the center of the hull. The pilot (prototype) vehicles showcased their engine exhaust grilles at the midway point of the hull sides but this was changed for production models where the grille now sat more forward. Pilot vehicles also showcased a muzzle brake which was not included on production forms.
Dimensions of the Hummel included a length of 7.17 meters, a width of 2.97 meters and a height of 2.81 meters. Weight was 23 tons. The operating crew numbered six and included the driver as well as five personnel to attend to the functions of the gun. While armored to an extent, the Hummel was only protected against small arms fire and artillery spray. For communication, the Fu.Spr.Ger.1 series radio set was installed. Drive power was derived from a Maybach HL120 TRM V-12 liquid-cooled, gasoline-fueled engine developing 265 horsepower at 2,600rpm. Road speeds reached 15.5 miles per hour with ranges out to 133 miles on roads (cross-country was limited to just 83.7 miles of distance).
The superstructure used by the gunnery crew only offered protection along the front, rear and sides leaving the top open for more operating space. This exposed the personnel to all manner of battlefield dangers including artillery splinters, air attack and grenade assaults not to mention the elements in general (a mesh covering could be affixed to help keep grenades out and tarp was typically used to battle the elements). Winter periods were particularly rough on the exposed crew. The driver, seated in the hull, was the only crewman afforded any sort of meaningful protection. The fixed nature of the superstructure meant that the entire vehicle had to be turned in the direction of fire. When made ready-to-fire, the rear doors of the superstructure were left open to provide ample space for the crew to enact the loading / reloading process.
The main gun consisted of the 15cm (150mm) sFH 18/1 series field howitzer offering a range out to 14,630 yards through primarily in-direct fire. Gunnery crews had access to a mix of ammunition types including the standard High-Explosive (HE) and Armor-Piercing (AP) rounds in addition to demolition and smoke rounds. In this way, the Hummel could operate as a dedicated artillery vehicle, a tank-killer, engineering vehicle or support vehicle. The main gun was locked to the glacis plate during travel by a standard A-frame assembly. Once unlocked, the gun mounting allowed for traversal of 15-degrees left-or-right from centerline of the vehicle. For secondary fire, the typical 7.92mm MG34 machine gun was fitted and 600 rounds were afforded to this weapon. It would prove vital in defending the vehicle against infantry assaults and attacks from low-flying aircraft.
From the outset, the Hummel was intended as an interim SPA pending delivery of a more dedicated, purpose-built form. However, 714 units later, the Hummel became a frontline fighting vehicle pressed into lengthy service, seeing combat into 1945. The gun carriers were arranged in batteries of six and recorded first-actions in July of 1943 during "Operation Zitadelle" (the famous Battle of Kursk). About 100 were in service by this time. In early-1944, the series saw a redesign to the front glacis plate section, revealing a new driver's compartment assembly that extended across the full-width of the hull and created more internal space for the radio station. Late-war models lacked the mufflers seen in early-war forms and had shrouds set over the grilles. Because of the rubber shortage witnessed across Germany, the original return rollers were also completed as all-steel.
Beyond the SPA Hummel model was the "Munitionstrager Hummel" which was nothing more than a gun-less variant of the Hummel developed as an ammunition resupply / carrier vehicle. One-hundred fifty of these were completed before the end of the war and these retained the ability to be converted back into their fighting forms as needed.