Lewin 1946 is often cited, but I'm not convinced. Also earlier papers may not always describe action research in its accepted sense. So when did we see a paper that would relate to what could be understood as action research?
German-American Kurt Lewin's 1946 ‘Action research and minority problems’ — in S Kemmis and R McTaggart (eds, 1990): 27–40 — is seen as coining the term. He certainly did see action research as collaborative and practical, social justice and policy-related. But UK Tavistock institute researchers were applying psychoanalytic approaches around the same time. Reg Revans would soon coin the term 'action learning'. Later, in the 1960s and 1970s, more emancipatory, not simply self-conscious and reflective, approaches bloomed in action research. Take a look at Yoland Wadsworth's easily accessible work — (2002) ‘We are one (river) and we are many (tributary streams)’ Action Learning Action Research, 7 (1), 3–28, at http://www.alara.net.au/files/ALARV7No1.pdf I think the point we discuss sometimes with students is that action research and learning has probably existed a very long time in partial forms but over the last 50 years it has been identified and described and developed as a particular approach and pursued. I think that Yoland's work is very useful.