Definitely not. AA's last domestic DC-3 was retired in 1949. Once the Convair 240s came along AA got rid of the 3s quite rapidly--well before any of the other then-"Big 4" carriers (EA, TW, UA).
American Overseas kept a couple in service (for Helsinki flights) until the '50 sale of AOA to Pan Am.
LGA-MDW in 1951 would definitely have been a DC-6 route, as the Convairs were used for shorter hauls and smaller markets.
