Good luck getting that B29 to 35k, the climb rate was at about 200fpm when I was at 27k with 75% fuel and 40 500lbers.
For 35k you need to take a light bomb load and less fuel. The practical limit for a full bomb mission is about 25-30k.
A 100% fuel, 40x500lbs B-29 takes 40 minutes to go to 20K, 54 minutes to reach 25 and 74 minutes for 30k (you still have ~90 minutes of fuel left at that altitude)