This happens only once so far in the anime and Manga, and it is in the Tartaros arc (the last arc of the Anime currently, as it is on Hiatus), during his fight against
Mard Gear Tartaros
During this fight
Natsu and Grey are generally losing against Mard Gear's Full power. Eventually, Mard Gear uses his ultimate move, a curse meant to kill even Zeref, and both Natsu and Grey get hit by it and are believed dead for a short time. It is then revealed that Gray was able to counter it using his newly gifted Devil Slayer Ice Magic, and is rendered mostly incapacitated for doing so. Natsu gets enraged and enters Dragon force without eating anything and for a short while overwhelms Mard Gear, which shows that he indeed fully achieved Dragon force, since he and Grey together were unable to do much but take a beating before this.
The event itself happens in chapter 411, which should be episode 263 according to the wiki,