Javelin Attack Speeder
The Javelin Attack Speeder was the Land Speeder's older brother, dating back to the Legiones Astartes during the Horus Heresy. As such, they have become an almost irreplaceable relic, and the ability to manufacture the craft as of M41 (thanks to a certain mishandling and hoarding of two Adepti) has had long since been lost.
apparently, the spiritual liege managed to locate an STC of the thing while on a blackstone fortress controlled by the Red Corsairs
The Javelin was larger and more heavily armored than the common pattern of Land Speeder, its advanced gravitic nullification allowed it to mount an array of heavy weaponry more akin to a tank destroyer than a nimble Land Speeder, allowing them to make pinpoint strikes on enemy armor or infantry with devastating Lascannons or Missile barrages. Those Chapters of Space Marines who date back to the days of the Heresy may still possess a few Javelins if they are fortunate enough.
Basically, if one considers the Jetbike to be a flying Assault Bike, then the Javelin is a flying Attack Bike.
History
The Javelin, as mentioned, used technology that has since been lost by a mountain of Administratum red tape. The technology, originating from the Dark Age of Technology, was first designed and manufactured for the Space Marine Legions by the Mechanicum during the Emperor's Great Crusade near the end of the 30th Millennium. Its late production and the high technology needed to craft these vehicles made them quite rare even in the waning years of the Great Crusade, and moreso during the 41st Millennium. Only Chapters dating back during this era have access to a few of them at most.
On the other hand, the Javelin is also available in small numbers to the Chaos Space Marine Traitor Legions and Warbands that trace their lineage back to the time of the Horus Heresy.
Model history
In the real world, the Javelin's history begins way back in the early days of Warhammer 40,000 (after Warhammer 40,000: Rogue Trader but not by much), when White Dwarf had an article about scratch-building models from everyday objects. One example given was turning an old stick of deodorant into a speeder (with the addition of some plasticard and spare weapons from the bitz box). Forge World made their model in the grand Horus Heresy tradition of updating goofy things from the late '80s and early '90s.
Weaponry
As previously mentioned, the Javelin is a far more heavily armed variant of the Land Speeder. The Javelin is armed with either a set of twin-linked Cyclone Missile Launchers or Lascannons as its main weapons. These weapons are located on the vehicle's flanks. The Javelin is also armed with a pintle-mounted Heavy Bolter that is used for close-range protection from enemy infantry. The pintle-mounted Heavy Bolter can be replaced with either a Heavy Flamer or a Multi-Melta. The Javelin can also be outfitted with a searchlight and up to two Hunter-Killer Missile Launchers for added firepower.
If you want to have a fast attacking tank killer, take these.