This is a clinical study for adult patients who have recently been diagnosed with acute myeloid leukemia or AML. AML is a type of cancer. It is when bone marrow makes white blood cells that are not normal. These are called leukemia cells. Some patients with AML have a mutation, or change, in the FLT3 gene. This gene helps leukemia cells make a protein called FLT3. This protein causes the leukemia cells to grow faster. For patients with AML who cannot receive standard chemotherapy, azacitidine (also known as Vidaza®) is a current standard of care treatment option in the United States. This clinical study is testing an experimental medicine called ASP2215, also known as gilteritinib. Gilteritinib works by stopping the leukemia cells from making the FLT3 protein. This can help stop the leukemia cells from growing faster. This study will compare three different treatments. Patients are assigned to one of these three groups by chance: an experimental medicine gilteritinib, a different medicine called azacitidine, also known as Vidaza®, or both medicines (azacitidine and gilteritinib) together. The clinical study may help show which treatment helps patients live longer.