XL-Calibur is a hard X-ray (15-80 keV) polarimetry mission operating from a
stabilised balloon-borne platform in the stratosphere. It builds on heritage
from the X-Calibur mission, which observed the accreting neutron star GX 301-2
from Antarctica, between December 29th 2018 and January 1st 2019. The
XL-Calibur design incorporates an X-ray mirror, which focusses X-rays onto a
polarimeter comprising a beryllium rod surrounded by Cadmium Zinc Telluride
(CZT) detectors. The polarimeter is housed in an anticoincidence shield to
mitigate background from particles present in the stratosphere. The mirror and
polarimeter-shield assembly are mounted at opposite ends of a 12 m long
lightweight truss, which is pointed with arcsecond precision by WASP - the
Wallops Arc Second Pointer. The XL-Calibur mission will achieve a substantially
improved sensitivity over X-Calibur by using a larger effective area X-ray
mirror, reducing background through thinner CZT detectors, and improved
anticoincidence shielding. When observing a 1 Crab source for $t_{\rm day}$
days, the Minimum Detectable Polarisation (at 99% confidence level) is
$\sim$2$\%\cdot t_{\rm day}^{-1/2}$. The energy resolution at 40 keV is
$\sim$5.9 keV. The aim of this paper is to describe the design and performance
of the XL-Calibur mission, as well as the foreseen science programme.
