The TIE Droid was initially developed by General Arndall Lott, who based his idea on the slaving experiment of the Katana Fleet and the droid fighters used by the Separatists. It was then given to Sienar Fleet Systems engineers to make into a reality. Sienar based what would soon be known as the TIE Droid on their original TIE Fighter, borrowing many of the elements the TIE series was known for and incorporating it into the new fighter.
If anything, the TIE/D was the epitome of Imperial fighter design philosophy. It was cheap, expendable, had a small target profile, and could be churned out in huge numbers. Without the need to spend credits on training pilots, the TIE/D was actually cheaper in the long run than the original TIE Fighter.
Despite being cheaper, the TIE Droid is actually tougher and faster than the venerable TIE Fighter. Sienar took the standard command pod and added additional armor and made it so the rectangular wing panels had an adjustable pitch. The TIE Droid was also able to beat the fearsome TIE Interceptor in atmospheric speed. Many of these improvements were possible because Sienar didn't have to take the life of a pilot into consideration.
Sienar made a limited number of the TIE Droids to test against New Republic fighters. Piloted by the Cybot Galactica Ace-6 droid brain, the TIE Droids proved to be no match for living pilots despite their superior numbers. A lack of shields and a tendency to be outwitted by the strategies of living pilots proved to be its downfall. The project was quietly canceled as Sienar focused on the next generation of the TIE series.
It would not be until the Xen'Chi Invasion that the TIE Droid project would be resurrected, but this time by the secret organization known as Oremin. Seeking ways to bolster the Empire's dwindling forces, Reng Kasr put his scientists to work on ways to fix the flaws in the TIE/D design, which he felt still had a lot of potential.
The TIE Droids were soon equipped with shields, much as the Empire had done to the TIE Interceptor. To improve the droid's A.I., Oremin copied memory engrams of their top pilots into a new droid brain of their own creation, giving the ships a form of semi-sentience and vastly improving their tactical abilities.