I was reading up on diffraction and in the textbook "Fundamentals of the Physical Theory of Diffraction" by Ufimtsev, the foreword was written by Kenneth Mitzner (who worked on the B-2 for Northrop), in which Mitzner said (in the quote "PTD" refers to Ufimtsev's physical theory of diffraction):

At Northrop, where I worked on the B-2 project, we were so enthusiastic about PTD that a co-worker and I sometimes broke into choruses of "Go, Ufimtsev" to the tune of "On,Wisconsin." At both Lockheed and Northrop we referred to PTD as "industrial-strength" diffraction theory...

So it seems that for the B-2, Northrop was using Ufimtsev's methods to numerically approximate RF scattering. What's interesting is that in Rebecca Grant's book "B-2 Spirit of Innovation", she described about how, prior to the B-2 development, Northrop went about designing their XST aircraft (ie. the competition that led up to the F-117) without the use of Ufimtsev's work, nor the benefit of the ECHO-1 software of Denys Overholser (the mathematician at Lockheed).

So I was wondering what was Northrop using at that time (pre-B-2 devlopment) for their numerical approximation of RF scattering in their LO design work? Grant had written that Northrop relied on their extensive experience in empirical testing of RCS of actual USAF aircraft in their design work, but she doesn't mention whether Northrop did or did not have a computational method to approximate RF scattering as an adjunct to their modeling and range testing.

During this timeframe (1960s-70s) there was an alternative method for numerical RF scattering approximation based on the work of Joseph Keller on the geometric theory of diffraction (GTD) which he developed in the early 1950s, but I haven't been able to find any traces of a possible link between Keller's work and Northrop's pre-B-2 LO design efforts.

So does anyone have any idea if Northrop did or did not use any computational method for RF scattering in their pre-B-2 work?