When were wheeled suitcases invented?

Rolling suitcases were invented in 1970, when Bernard D. Sadow applied for a patent that was granted in 1972 as United States patent 3,653,474 for "Rolling Luggage". The patent application cited the increase in air travel, and "baggage handling [having] become perhaps the single biggest difficulty encountered by an air passenger", as background of the invention. Sadow's four-wheeled suitcases, pulled using a loose strap, were later surpassed in popularity by rollaboards, suitcases that feature two wheels and are pulled in an upright position using a long handle, and were invented in 1987 by US pilot Robert Plath.