Correct answer is... 43 m. It is equal to 140 ft. Wingpan is calculated as stright line from one wingtip to another, so shape of the wing is not taken as factor during measurement. The biggest civil aircraft is 88 m (290 ft) wide, so Douglas XB-31 has 48% percent of it's length. Typical jet airliners are about 34 m wide, so in the comparision Douglas XB-31 has 51% of aircrafts like B737 or A320.
Disclaimer: Our articles and data connected to wingspan of Douglas XB-31 are based on public data from various sources. We are very picky to select an reliable data providers, but we cannot guarantee that everything is 100% error-proof. We recommend for any serious purpose - as a rule of thumb - to request offical data about wingspan from manufacturer of Douglas XB-31.