The calculated distance (air line) between Düsseldorf-Pempelfort and Dortmund is approximately 35 miles respectively 56 kilometers.
By car or train, the actual journey to Dortmund is certainly longer, as only the direct route (as the crow flies) between Düsseldorf-Pempelfort and Dortmund has been calculated here.
Air line (approximately)
56 kilometers
30 nautical miles