From the Document: "[T]he Air Force is in the process of replacing its fleet of 396 KC-135 Stratotanker refueling aircraft, built in the 1950s and 60s, and 59 KC-10 Extenders, which entered service in 1981. Recent announcements indicate that the planned replacement program is changing significantly from its original form, which Congress may consider in evaluating the FY2022 defense budget requests. The Air Force originally envisaged replacing the current tanker fleet in three stages.  An initial acquisition of 179 new aircraft procured through the KC-X competition (won by the Boeing KC-46A) would replace roughly one-third of the KC-135 fleet.  A further 179 tankers were projected to be procured in a second solicitation called KC-Y; initially projected as a new competition based on what aircraft were available at the time, it was subsequently recast as a continuation of KC-46A procurement.  A third program, KC-Z, was to be a replacement for the KC-10 fleet, a larger tanker than the KC-46. Subsequently, the Air Force dropped plans for the KC-Z, envisioning it instead as a third tranche of KC-46s."
CRS Insight, IN11537
Congressional Research Service: https://crsreports.congress.gov/