I have been spending a few hours now trying to Flatten this part on the 2017 Standard Version. I do not have a premium version and can therefore not use the flattening command. When trying to use insert Bend and selecting on of the the bottom edges it always displays: "Cannot select this type of edge. I do not really understand why this happens. The plan is to flatten the part and export it as a DXF so I can make a cutting Jig for it.
Your best bet once you have your shape is to use the lofted bend feature. I just used your two end faces to create a sketch & then converted entities using the outside arc in each sketch & then did a lofted bend. I then used your existing sketch for the cuts in the new body & it flattened OK.