Certainly before the war- when most of the operational aircraft actually fighting WWII were initiated- there were some remarkably casual rules of thumb involved. "Well, if our current bomber carries 1000 lbs 1000 miles at 200 mph, then this one should carry 1500 lbs 2000 miles at 250 mph." Those nice round numbers are a giveaway (I made them up to illustrate my point, but if you look at the early planning you'll see such things).
And specs usually ask for something not necessarily quite (as of yet) within reach, because who wants to get last year's best warplane?
Of course, as hinted at by somebody (A-26/A-14), a few years can change things rather dramatically- witness the B-29 being considered a medium bomber in Korea!