Sikorsky R4B. Made first ever flight in Jan 1942. USAAF (pre Air-Force) ordered 100 of them. First combat duty in May 1944.Here's the one @ AF Museum in Dayon OH.https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195868/sikorsky-r-4b-hoverfly/