A possible synthesis would be MeMgX addition to benzhydrylnitrile followed by the reduction of the intermediate imine with NaBH4 (see Diphenidine).
Starting from P2P [103-79-7], 1,1-Diphenylacetone [781-35-1] (see Diphenadione).
Schmidt D, Hüller H, Amon I, Peters R. Zur Pharmakologie einer Serie von Diphenylaminopropanderivaten [On the pharmacology of a series of diphenylaminopropane derivatives]. Pharmazie. 1973 Oct;28(10):677-80. German. PMID: 4783556.