Prolonged Casualty Care (PCC) is a military adaptation aimed at providing pre-hospital care in austere settings when evacuation is delayed or even impossible. Current lack of standardized medical equipment and size/weight restrictions of military packs during dismounted operations hinder effective PCC. We sought to design a standardized, practical, and effective prolonged field care kit (PFAK) to enable widespread implementation of PCC. We reviewed Joint Trauma System Clinical Practice Guidelines to generate a list of potential contents of the PFAK. We obtained Institutional Review Board (IRB) exemption and then conducted stakeholder surveys of combat casualty care experts across the Joint Trauma System using a modified Delphi survey approach. We established a civil-military working group that provided in-depth qualitative feedback on the PFAK contents and provided an initial design of a long-range medical rucksack (LMR) to house it. Responses were analyzed using mean rank scores to help determine initial components of the PFAK. Tactical subject-matter experts tested and evaluated the PFAK and LMR prototype in austere conditions to refine the design. Review of the PCC Clinical Practice Guidelines generated 49 medications and 301 potential supplies as potential PFAK contents. The first Delphi survey was sent to 100 stakeholders (overall response rate of 60%). After the first survey, contents were narrowed to a list of the most essential 27 medications and 105 other components. Iterative prototypes of the PFAK and LMR were tested to determine ergonomics, portability, flexibility, and equipment compartmentalization to facilitate use in emergencies. The prototype was optimized to address the clinical, logistical, and tactical requirements of PCC across a variety of platforms and environmental conditions. Given the changing battlefield environment, efficient and effective PCC will play an increasingly important role in the management of combat trauma. The PFAK can meet this need by providing a practical and standardized resuscitation kit generated by expert military and trauma personnel consensus, carried conveniently in the LMR.