I'm watching Girls Last Tour 06 and one thing bothers me about Ishii's airplane: it's far too large for the mission. She wanted to have a range of 2,000 km, or about 1,080 nautical miles (nm). But that is a range well within reach of a relatively basic single-engine airplane as we know them today. For example, Cessna 182 has a range of 915 nm, with 522 lbs of standard fuel. However, it can carry more, being a 4-seater. If one were to install a ferry tank, even staying within the design's certified gross weight, and allowing for 300 lbs for pilot and luggage (survival gear), 323 lbs would be available, extending the range well over 1,400 nm. I'm sure that it would actually be greater, because the Breguet Equation is not linear.

So, why didn't Ishii build a Cessna 182 instead of the monster we're shown? I think she didn't tell everything to Chito and Yuuri. What she really was after was not just the range, but altitude. While you can cross 1,400 nm in a 182, it borders on suicide without a meteorological support. But if you could go 35,000 ft up, you can get over just about any weather, and hopefully find a spot to descend on Ishii's mission.

P.S. This airplane, ANT-25, is about the same size as Ishii's, and it flew from Russia to U.S. over the North Pole:

Note that the wing tapers quite sharply on it. That permits most of the weight to be carried inboard, yet capture the benefits of high aspect ratio. The root thickness is also much greater than in Ishii's design. It's as if A.N.Tupolev knew something about spars breaking.