Somewhere in my house I have a United States Army Air Force manual titled "Weather Flying" that belonged to my father that he kept from when he was in basic flight training dated 1941.
Although I must paraphrase here I clearly remember some of the statements in this manual convening Thunderstorm penetration. It went something like this;
1. When approaching a line of heavy thunderstorms fly toward the area of heaviest rain.
2. Avoid areas with frequent and heavy lighting.
3. Fly aircraft at reduced airspeed.
4. Lower seat to lowest position possible.
5. Turn all cockpit lights to full bright.
6. Secure all lose items in cockpit and cabin.
8. If possible restrict aircraft altitude to below the freezing temperate level.
9. Enter thunderstorm at lowest altitude while still keeping a safe level above terrain.
Then the manual went on to make statements such as, safe flight in thunderstorm can be completed, try to avoid green clouds as that is an indication of hail, expect heavy water leakage into cockpit, etc
Well I can certainly attest to the water leaking into the cockpit from my old DC-3 days, and not even close to any thunderstorm, just good rain.
When I first started my aviation career as a pilot I flew with a lot of World War II era pilots. As 411A said, until the jet-upset accidents started happening most of those guys considered thunderstorms just another cloud.
I also flew thunderstorm research many, many years ago in a (believe it or not) Twin Comanche. However, I never actually penetrated any cells, not on purpose anyway, I flew around the bases recording data and firing off flares. It was a study conducted by the US Air Force, NOAA and the University of Oklahoma. The aircraft used were, Twin Comanche (me), Cessna 401, T-28, F4 and a modified B-57 Canberra. There was a F-100 used as well, however, it flew through a heavy area of hail at very high speed and although the pilot was able to land the aircraft it was written off.
Trust me, we have thunderstorm super-cells in Oklahoma that top FL60.0 +. One does not mess with them. Matter of fact one should not mess with any thunderstorm.