Obiltoxaximab (Anthim(®), ETI-204) is a monoclonal antibody that is being developed by Elusys Therapeutics and the US Department of Health and Human Services' Biomedical Advanced Research and Development Authority for the prevention and treatment of inhalational anthrax due to Bacillus anthracis. Obiltoxaximab has been designed to neutralize the free protective antigen of B. anthracis, thereby inhibiting the lethal effects of anthrax toxins. In March 2016, intravenous obiltoxaximab was approved in the USA for the treatment (in combination with appropriate antibacterial drugs) and prophylaxis of inhalational anthrax. Obiltoxaximab is being developed under the US FDA Animal Rule, in which marketing approval is based on its efficacy in relevant animal models and safety in phase I studies in healthy human volunteers. An intramuscular formulation of obiltoxaximab has also been evaluated in animal studies and a phase I study in healthy human volunteers. This article summarizes the milestones in the development of obiltoxaximab leading to this first approval for the treatment and prevention of inhalation anthrax.