What is the X_B file format?
An .x_b file is a Parasolid binary transmit file - a compact binary encoding of a 3D solid model produced by the Parasolid geometry kernel, which powers major CAD applications including SolidWorks, Siemens NX, and Solid Edge.
Parasolid stores geometry as precise boundary-representation (B-rep) solids: exact mathematical surfaces, edges, and faces rather than triangle meshes. This makes .x_b files suitable for downstream measurement, modification, and manufacturing, unlike mesh-based formats such as STL.
The file header opens with a B sentinel byte, distinguishing the binary variant from its text-based counterpart .x_t (which begins with T). The header also encodes the Parasolid schema version, enabling receiving applications to verify compatibility. The file body contains serialized B-rep entities - solids, shells, faces, edges, vertices, and assembly instances - in Parasolid's cross-platform binary serialization.
.x_b and .x_t carry identical geometric information; the binary form is more compact and faster to parse, while the text form is human-readable. Both are widely used as neutral exchange formats between Parasolid-licensed CAD systems. The older filenames for the pair are xmt_bin (.x_b) and xmt_txt (.x_t). Siemens does not publish the full binary specification publicly; access requires a Parasolid SDK license.
Security & safety
RISK: LOWAn X_B file contains passive 3D geometry data - no executable code. Opening one is safe. Main non-malicious risk: exact B-rep geometry exposes all design dimensions to the recipient (IP exposure). Large assemblies may be slow to process. Verify the true extension of any file from an untrusted source.
Format details
in a nutshellPrograms that open X_B files
Windows5 apps
eDrawings Viewer
Free Free Dassault viewer - open the .x_b to rotate, section, and measure. Familiar to SolidWorks users. 
CAD Assistant (Open Cascade)
Free Free offline viewer/converter - open the .x_b to view, measure, and export (e.g. STL/STEP/glTF). Best free option for Parasolid binary files.
Autodesk Fusion 360
Freemium Upload/insert the .x_b to import the solid body for editing or STL export. Free for personal/hobby use.
SolidWorks / Solid Edge / NX
Paid These are Parasolid-based CAD systems - File > Open imports .x_b natively with full fidelity. 
macOS2 apps
Linux2 apps
Parasolid (Linux SDK)
Paid Siemens Parasolid SDK/kernel for Linux - developer/OEM tool, not an end-user app. Technical details
deep spec| Format type | 3D CAD solid model - Parasolid kernel binary "transmit" file using boundary representation (B-rep) geometry |
| Header sentinel | `B` byte (ASCII 0x42) at file start, distinguishing binary from text variant `.x_t` (which starts with `T`) |
| Geometry representation | Exact mathematical B-rep surfaces, edges, and faces - not a polygon mesh; suitable for measurement and machining |
| Schema version | Version number encoded in file header (e.g. "Parasolid schema 35.0"); current schema 36.x |
| Binary encoding | Parasolid cross-platform binary serialization (architecture-independent; not conventional big/little-endian) |
| Typical file size | 50 KB - 500 MB (single part to large assembly) |
| Text counterpart | `.x_t` (also `xmt_txt`) carries identical geometry in ASCII form; `.x_b` (also `xmt_bin`) is more compact and faster to parse |
| Assembly support | Supports multiple component bodies and placement transforms within one file |
| MIME type | `model/vnd.parasolid.transmit.binary` (registered IANA MIME type) |
| License requirement | Parasolid SDK license required to read/write; binary specification not publicly published by Siemens |
| CAD tool support | Supported by all Parasolid-licensed applications: SolidWorks, Siemens NX, Solid Edge, Autodesk Inventor, Fusion 360 |
| Comparison with STEP | Unlike [STEP](/format/step) (.stp), `.x_b` is proprietary and requires a Parasolid license; STEP is the ISO neutral exchange format |
| Released | Parasolid kernel since 1988 (Shape Data/Unigraphics); .x_b/.x_t transmit files established in early 1990s |
| Latest version | Parasolid schema 36.x (current; version tracked in file header) |
| Specification | plm.sw.siemens.com |
X_B conversions
Community Q&A
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