The B-47 was a very clean machine and was quite fast on anemic engines, 6,000 lb thrust J47. The B-47 was never updated engine wise. I was wondering what the B-47 could have done with better engines like the J57 used in the B-52. The J57 had about twice the thrust of the j47. The J57 was slightly bigger and a bit over a 1000 lbs heavier. If it was not too hard to strengthen the wings to hold the weight and power, the B-47 may have been capable of high subsonic speeds. With the added speed and altitude the engines could provide, the B-47 could have been a continuing threat until the Mig-21 arrived in 1959. I think the Mig-19 would have had a hard time intercepting it. Of course the AF was only interesting in getting the B-52 at that time and there was no thought of adding money to the B-47 program.