No, you would be right to think that it has never been proven that house flies (Musca domestica) did ever infect humans with Influenza A Virus, but still it is a possibility that scientists are very much aware of.
The first clue that house flies might be a potential vector for the Influenza A Virus appeared in 1985 when a report appeared that discussed how in early 1983 an Influenza A(H5N2) virus of low virulence infected chickens in Pennsylvania (USA), but that virus turned virulent later that year and caused high mortality in poultry. Buried in the report was that fact that ‘the ‘virus was also isolated from house flies in chicken houses’.
House flies have a very varied diet and might feed off infected, diseased or deceased animals. Influenza A Virus might therefore be present in the alimentary tract of these house flies and laboratory experiments indicate that the house fly can be a potential carrier of Influenza A Virus.
All this suggests that there is yet another unexpected host of Influenza A Virus.
 Bean et al: Characterization of virulent and avirulent A/chicken/Pennsylvania/83 influenza A viruses: potential role of defective interfering RNAs in nature in Journal of Virology - 1985
 Wanaratana et al: The potential of house flies to act as a vector of avian influenza subtype H5N1 under experimental conditions in Medical and Veterinary Entomology - 2011
 Nielsen et al: Persistence of low-pathogenic avian influenza H5N7 and H7N1 subtypes in house flies (Diptera: Muscidae) in Journal of Medical Entomology - 2011