As far as I know, the B-50 fitted in somewhere between the B-29 and the B-52...The B-29 had Wright R-3350's...As far as the Lancaster went, it wasn't designed [ at any stage ] to carry an A-Bomb, nor were it's latter variants ; the Lancastrian, the York, both of which were essentially transport aircraft, nor the Lincoln or the Shackleton. - However, the designs for the Lincoln were laid down in 1943 [B14/43] and first flew in June 1944. - It was capable of 35.000 ft ceiling, with Merlin 68's or 85's, and with longer span wing with a higher aspect ratio, a lengthened fuselage and new nose, a modified bomb bay and strengthened landing gear....They were in service until 1955, when the V-Bomber program started, the first UK Nuke-Bombers. - Essentially though, the Lincoln was simply just a modified Lancaster, and with different engines could maintain a ' Nuclear Altitude'...The Lancaster still, IMHO, could've, if required, handle an A-Bomb...The reason the Brits didn't rush around to build a Nuke-bomber before the 'V series', was probably post-Japan, they didn't see a requirement, not until the Cold War anyway...the last Lancasters remained in service [ with Canada ] until 1964, which really is a testament to their popularity....We shall have to differ in our opinions on this, gentlemen, because I'm adamant that the Lancaster could've done-it, but I do concede the B-29 was a great aircraft, teething-troubles and all and certainly did sterling service both as a conventional nuke bomber....