Accelerating Defense Innovation Act of 2019 This bill establishes a pilot program within the Department of Defense (DOD) to grant money to certain small businesses and modifies provisions related to DOD funding of small businesses through Small Business Administration programs. Specifically, the bill creates the Domestic Investment Pilot Program, which allows DOD and service acquisition executives for each military department to make a Small Business Innovation Research (SBIR) program award to small businesses that are majority-owned by multiple domestic venture investors. However, no more than 15% of DOD SBIR funds may be awarded to such small businesses. In addition, the bill doubles funding for the Defense Research and Development Rapid Innovation Program, which is a competitive, merit-based program designed to accelerate the deployment of technologies developed through SBIR projects. The bill also allows DOD to award Small Business Technology Transfer (STTR) program funds to accelerate the creation of small businesses and the commercialization of research innovations. Further, the bill authorizes discretionary technical and business assistance through the SBIR and STTR programs for the purpose of aiding small businesses with cybersecurity decision-making, problem-solving, and development and commercialization of new products and processes. Lastly, DOD, in consultation with the military departments, must establish at least three joint reserve detachments at locations of the Defense Innovation Unit to (1) support engagement and collaboration with commercial innovation hubs, and (2) accelerate the transition and adoption of commercial technologies for national security purposes.